Boasting supporter with volume trouble (7)
I believe the answer is:
bravado
'boasting' is the definition.
(I've seen this before)
'supporter with volume trouble' is the wordplay.
'supporter' becomes 'bra' (a bra gives support to breasts).
'with' says to put letters next to each other.
'volume' becomes 'v' (abbreviation for volume in book titles).
'trouble' becomes 'ado' (both can mean bother or commotion).
'bra'+'v'+'ado'='BRAVADO'
(Other definitions for bravado that I've seen before include "great self-confidence" , "Impressive boldness" , "Swag" , "Unjustified assertion of confidence" , "showing off" .)