Couple with wife very large in volume (7)
I believe the answer is:
twosome
'couple' is the definition.
('twosome' can be a synonym of 'couple')
'wife very large in volume' is the wordplay.
'wife' becomes 'w'.
'very large' becomes 'os' (abbreviation for oversize).
'in' means one lot of letters goes inside another.
'volume' becomes 'tome' (I've seen this before).
'w'+'os'='wos'
'wos' placed within 'tome' is 'TWOSOME'.
'with' acts as a link.
(Other definitions for twosome that I've seen before include "Couple" , "Pair, duo" , "Item" , "game of golf" , "match" .)