Hold up back of trousers with belt or elastic (7)
I believe the answer is:
bolster
'hold up' is the definition.
(I've seen this before)
'back of trousers with belt or elastic' is the wordplay.
'back of' indicates one should take the final letters.
'with' is a charade indicator (letters next to each other).
'elastic' is an anagram indicator (the anagram stretches or twists into a new form).
The final letter of 'trousers' is 's'.
'belt'+'or'='beltor'
's' after 'beltor' is 'beltors'.
'beltors' is an anagram of 'BOLSTER'.
(Other definitions for bolster that I've seen before include "Reinforce; a cushion" , "Large pillow" , "Strengthen - long pillow" , "Provide reinforcement" , "Prop up with long pillow" .)