Only having a drink on coming in (5)
I believe the answer is:
alone
'only' is the definition.
(synonyms)
'a drink on coming in' is the wordplay.
'a drink' becomes 'ale' (type of beer).
'coming in' means one lot of letters goes inside another.
'ale' going around 'on' is 'ALONE'.
'having' acts as a link.
(Other definitions for alone that I've seen before include "Without friends" , "Without others present" , "without partner?" , "Solo" , "Isolated; exclusively" .)