This is a difficult question to answer insofar as it draws into question the ultimate subjectivity of trademark law. You seem to have a very good grasp on the issue - the addition of a distinct elements to the same generic or descriptive term. Often this will be permissible. Often it will not. In regard to your example, it may come down to (1) how diluted and distinctive the common term is on the register and (2) how distinctive the added element is on the register.
In your example "pro" is may be deemed to be laudatory in nature and thus descriptive and, as such, not entitled to much protection. Altering it to "pal" may be sufficient to distinguish your mark apart from that already registered. But it may not.
What we like to do in these situations is to look on the register for how many marks include "CleanPro" or some other two to four word term after "clean" so that we can claim the mark is diluted and that the substitution of any other term should thus be entitled to registration.
If CleanPro is the only mark on the register so existing there is a higher likelihood your mark will get rejected as opposed to, for instance, if there is a CleanPro, CleanBee, CleanX, CleanPremium, etc. on the register all for the same goods such that the argument could be made that slight distinctions in the marks would be viewed by consumers as distinguishing between the goods or services provided thereby.
Hope that this manner of analysis helps. Good luck.
/Matthew H. Swyers/