Only having a drink on coming in (5)
I believe the answer is:
alone
'only' is the definition.
('alone' can be a synonym of 'only')
'a drink on coming in' is the wordplay.
'a drink' becomes 'ale' (ale is a kind of drink).
'coming in' indicates putting letters inside.
'ale' placed 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" .)