Jack, first to open bottle, is merry (6)
I believe the answer is:
jovial
'merry' is the definition.
('jovial' can be a synonym of 'merry')
'jack first to open bottle' is the wordplay.
'jack' becomes 'j' (playing card abbreviation).
'first to' says to take the initial letters.
'bottle' becomes 'vial' (vial is a kind of bottle**).
The first letter of 'open' is 'o'.
'j'+'o'+'vial'='JOVIAL'
'is' is the link.
(Other definitions for jovial that I've seen before include "Happy and cheerful" , "Jolly, good-humoured" , "Good-humoured, cheerful" , "Hearty and cheerful like Jupiter" , "Full of cheerful good humour" .)