Constituent in tight top and whopping rear (8)
I believe the answer is:
integral
'constituent' is the definition.
(I've seen this before)
'in tight top and whopping rear' is the wordplay.
'top' suggests taking the first letters.
'and' is a charade indicator (letters next to each other).
'whopping' becomes 'large' (similar in meaning).
'rear' says the letters should be written in reverse (I've seen 'rearing' mean this (as in a horse rearing up)).
The initial letter of 'tight' is 't'.
'large' back-to-front is 'egral'.
'in'+'t'+'egral'='INTEGRAL'
(Other definitions for integral that I've seen before include "Necessary as part of a whole, like triangle" , "Formed of constituent parts" , "United" , "Altering (anag)" , "Forming part of the whole" .)