Drunk keeps line in happy tune (4)
I believe the answer is:
lilt
'happy tune' is the definition.
(I've seen this in another clue)
'drunk keeps line' is the wordplay.
'drunk' becomes 'lit' (I've seen this in another clue).
'keeps' indicates putting letters inside.
'line' becomes 'l' (used when specifying particular lines from a poem).
'lit' going around 'l' is 'LILT'.
'in' acts as a link.
(Other definitions for lilt that I've seen before include "Sing happily" , "Buoyant motion" , "Tune's gentle rhythm" , "Musical quality of voice" , "Cadence (in speech)" .)