Drunk on leaving this evening (5)
I believe the answer is:
tight
'drunk' is the definition.
(slang term)
'on leaving this evening' is the wordplay.
'leaving' is a deletion indicator.
'this evening' becomes 'to-night' (synonyms).
'tonight' with 'on' removed is 'TIGHT'.
(Other definitions for tight that I've seen before include "Drunk; restricted" , "Drunk, firm, concise" , "Sozzled; mean" , "corked?" , "Tense; secretive" .)