Forum contributor drinking port is an arse (9)
I believe the answer is:
posterior
'an arse' is the definition.
(I know that arse can be written as posterior)
'forum contributor drinking port' is the wordplay.
'forum contributor' becomes 'poster' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'drinking' indicates putting letters inside (inserted letters are consumed or drunk).
'port' becomes 'rio' (Rio de Janeiro is a port).
'poster' enclosing 'rio' is 'POSTERIOR'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for posterior that I've seen before include "Rear" , "Stern" , "later" , "Backside" , "Hindquarters" .)