Near agreement, backing a last drink (8)
I believe the answer is:
nightcap
'a last drink' is the definition.
'near agreement backing' is the wordplay.
'near' becomes 'nigh' ('nigh' can be a synonym of 'near').
'agreement' becomes 'pact' (pact is a kind of agreement).
'backing' shows that the letters should be reversed in order.
'pact' written backwards gives 'tcap'.
'nigh'+'tcap'='NIGHTCAP'
(Other definitions for nightcap that I've seen before include "Patching (anag.)" , "Old headgear" , "drink having sleepy effect?" , "Bedtime beverage" , "Head covering" .)