Immense hidden chaps (3)
I believe the answer is:
men
'chaps' is the definition.
(I've seen this before)
'immense hidden' is the wordplay.
'hidden' indicates the answer is hidden within the clue.
'MEN' is hidden within 'immense'.
(Other definitions for men that I've seen before include "Soldiers" , "Chess pieces" , "Adult males" , "Male humans" , "Human males" .)