Some slim men seem massive (7)
I believe the answer is:
immense
'massive' is the definition.
(I've seen this before)
'some slim men seem' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'IMMENSE' is hidden in the letters of 'slim men seem'.
(Other definitions for immense that I've seen before include "Massive" , "Very big, exceedingly great" , "Exceedingly large" , "Very good" , "Huge, very big indeed" .)