Build and item logic
So I find clockwork a very easy hero to time last hits with. I combo this with his rockets to last hit against an aggressive laner. Even if the opposing hero(s) aren't aggressive this constant bombardment really eats at non-str heros and burns any tangos or potions early, as well as helping any lane pushing.
I find that most of the time I'm not leaving lanes early with this hero so I max it first which also improves the cd which I can then spam view points or other lanes/enemy heros. The mana cost is minimal and you can more or less use it once a wave without depleting your mana.
It depends who you are facing whether or not you level cogs or ministun next. I prefer cogs though because if you can get blademail early, the ability to trap someone in for that entire duration is almost an automatic death if you get them alone.
I can normally make a full blademail by level 6/7 if I'm in a favorable lane. Once I get the blademail I look to then initiate a gank or catch my opposing hero unaware of the item. This is especially effective if their health is already low due to rockets. I really like to be overly aggressive in the use of my ult and thats why I choose cogs over stun for the more disabling feature-which makes the enemies play into your blademail.
Anywho, after bmail I prefer to go treads for the extra health, but phase I find just as good. I then will look to get either radience or scepter. If you are not farming well though or cannot get reaver before lvl 11/12 or so I would not try to make a radience. I might even consider a vanguard. Boots of Travel are never a bad thing to own and although are arguably too pricey and unnecessary on clock, I find they pair quite effectively.
Buy items that work on getting big, and helping team play. Clockwork is very flexible and I find I buy different depending on what role he needs to fill for the team. For example I've gone anywhere from Armlet to Scythe on him depending on the teams need and done well. If going true support I really recommend Vlads, Drum, Pipe and Necron.
*I completely understand any detractors to the preference of radiance in the build. Note before that I say this must be purchased early or completely abandoned. The damage and aoe combos nicely with clock's abilities (great for easy laning) but these advantages obviously lose importance immediately if you are not ahead of the curve in both experience and gold.