Students' union is full of extremely decorative posers (5)
I believe the answer is:
nudes
'posers' is the definition.
'students' union is full of extremely decorative' is the wordplay.
'students' union' becomes 'nus' (National Union of Students).
'is full of' indicates putting letters inside.
'extremely' suggests removing the centre.
'decorative' with its centre removed is 'de'.
'nus' enclosing 'de' is 'NUDES'.
(Other definitions for nudes that I've seen before include "Dunes (anag.)" , "Unclothed people" , "They're uncovered" , "People without clothes on" , "Revealing paintings?" .)