Close deal over a drink (8)
I believe the answer is:
nightcap
'a drink' is the definition.
(I know that nightcap is a type of drink)
'close deal over' is the wordplay.
'close' becomes 'nigh' (similar in meaning).
'deal' becomes 'pact' (I've seen this in another clue).
'over' says the letters should be written backwards.
'pact' back-to-front is 'tcap'.
'nigh'+'tcap'='NIGHTCAP'
(Other definitions for nightcap that I've seen before include "that may go to one's head?" , "Tot, at bedtime" , "A drink before sleep" , "Alcoholic drink taken at bedtime" , "drink having sleepy effect?" .)