Say no men can possibly hold it back! (7)
I believe the answer is:
mention
'say no' is the definition.
'mention' can be an answer for 'say' (mentioning is a kind of saying). I am not certain of the 'no' bit.
'men can possibly hold it back' is the wordplay.
'can possibly' becomes 'no' (I can't explain this - if you can you should believe this answer much more).
'hold' means one lot of letters go next to another.
'back' says the letters should be written backwards.
'no'+'it'='noit'
'noit' in reverse letter order is 'tion'.
'men'+'tion'='MENTION'
Can you help me to learn more?
(Other definitions for mention that I've seen before include "Make reference to" , "Briefly refer to (something)" , "state" , "Acknowledge" , "Passing reference" .)