Bill holds port back (9)
I believe the answer is:
Here is my best explanation:
'back' is the definition.
('posterior' can be similar in meaning to 'back')
'bill holds port' is the subsidiary indication.
'bill' becomes 'poster' ('poster' can be a synonym of 'bill'**).
'holds' is an insertion indicator.
'port' becomes 'rio'.
'poster' placed around 'rio' is 'posterior'.
(Other definitions for posterior that I've seen before include "Rear", "later", "Behind", "Stern", "Being behind", "Further back; bottom", "Back", "Hindquarters" and "Backside".)