Bill holds port back (9)
I believe the answer is:
posterior
'back' is the definition.
('posterior' can be similar in meaning to 'back')
'bill holds port' is the wordplay.
'bill' becomes 'poster' ('poster' can be a synonym of 'bill').
'holds' means one lot of letters goes inside another.
'port' becomes 'rio' (I've seen this before).
'poster' going around 'rio' is 'POSTERIOR'.
(Other definitions for posterior that I've seen before include "Rear" , "Stern" , "Hindquarters" , "Further back; bottom" , "later" .)