Jack, over bottle, becomes jolly (6)
I believe the answer is:
jovial
'jolly' is the definition.
('jovial' can be a synonym of 'jolly')
'jack over bottle' is the wordplay.
'jack' becomes 'j' (playing card abbreviation).
'over' becomes 'o' (cricket abbreviation).
'bottle' becomes 'vial' (vial is a kind of bottle**).
'j'+'o'+'vial'='JOVIAL'
'becomes' is the link.
(Other definitions for jovial that I've seen before include "Convivial" , "'Jolly, merry (6)'" , "Happy and cheerful" , "Full of cheerful good humour" , "Full of mirth" .)