Couple in book holding pig back (7)
I believe the answer is:
twosome
'couple' is the definition.
(I know that couple can be written as twosome)
'book holding pig back' is the wordplay.
'book' becomes 'tome' (a tome is a book).
'holding' means one lot of letters goes inside another.
'pig' becomes 'sow' (I've seen this before).
'back' says the letters should be written backwards.
'sow' in reverse letter order is 'wos'.
'tome' going around 'wos' is 'TWOSOME'.
'in' acts as a link.
(Other definitions for twosome that I've seen before include "match" , "game of golf" , "Item" , "Couple" , "Pair, duo" .)