Gentlemen in suits. Tops! (7)
I believe the answer is:
'tops' is the definition.
(I know this)
'gentlemen in suits' is the wordplay.
'gentlemen' becomes 'mm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'mm' put inside 'suits' is 'SUMMITS'.
Can you help me to learn more?
(Another definition for summits that I've seen is " tops".)