Personally, I don't see too much of a problem using clen during the cycle, except that the shakes might interfere with lifting in some people. The raise in metabolism from clen might make it harder to put on weight, but you should be putting on more LEAN weight this way, which it sounds like you want (otherwise you wouldn't even be thinking of clen). This is just my opinion though, and I can see benefits in leaving it for post-cycle.

If you want to use it during:
start clen the first week and cycle it 2 weeks on, 2 off. I would add 1 tab a day until you hit 100-200mcg depending on how comfortable you are with the shakes and headaches (feels more like migraines) you get from it. Then work back down the same way. Once you know your dose, you can start much closer to it and work up faster on future clen cycles. Continue clen through and even maybe past clomid-therapy to help use it's anticatabolic properties to keep more weight post-cycle.

If you want to use it only post cycle:
start it immediately following the last week of sust (so week 11) and run the same as above. This will help with retaining more weight when you come off.

Later