Gent's undies regularly removed for exercise (5)
I believe the answer is:
etude
'for exercise' is the definition.
(I have seen 'Musical exercise ' mean 'etude' so perhaps 'exercise' could also mean 'etude')
'gent's undies regularly removed' is the wordplay.
'regularly removed' means one should take alternating letters (letters removed at regular intervals).
'gents'+'undies'='gentsundies'
The alternate letters of 'gentsundies' are 'ETUDE'.
(Other definitions for etude that I've seen before include "short musical piece" , "Chopin study" , "Noted composition" , "Indian statesman" , "A short musical exercise" .)