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' indicates putting letters inside.
'ale' enclosing 'on' is 'ALONE'.
'having' is the link.
(Other definitions for alone that I've seen before include "Without friends" , "Without others present" , "without partner?" , "Solo" , "Isolated; exclusively" .)