Unhappy in boozer, about to hit the roof (4,2)
I believe the answer is:
blow up
'hit the roof' is the definition.
(both can mean to become angry)
'unhappy in boozer about' is the wordplay.
'unhappy' becomes 'low'.
'in' is an insertion indicator.
'boozer' becomes 'pub' (UK slang).
'about' says the letters should be written in reverse.
'pub' in reverse letter order is 'bup'.
'low' inserted within 'bup' is 'BLOW-UP'.
'to' is the link.
(Other definitions for blow up that I've seen before include "Expand" , "Detonate - enlarge" , "reduce to smithereens" , "Explode or lose temper" , "Begin to develop" .)