No gin served up? One whisky short? He may be devastated! (5,6)
I believe the answer is:
party animal
'he may be devastated' is the definition.
The definition suggests a singular noun but the answer is not.
'no gin served up? one whisky short?' is the wordplay.
'no' becomes 'nay' (archaic form 'no').
'gin' becomes 'trap' (I've seen this in another clue).
'served up?' says the letters should be written in reverse (in a down clue, going upwards).
'one' becomes 'I' (Roman numeral).
'whisky short?' becomes 'mal' (I can't explain this - if you can you should give a lot more credence to this answer).
'nay'+'trap'='naytrap'
'naytrap' reversed gives 'partyan'.
'partyan'+'i'+'mal'='PARTY ANIMAL'
(Other definitions for party animal that I've seen before include "socialite?" , "who enjoys meetings?" , "Republican elephant, for example?" , "Democrats' donkey, for example" , "I get out a lot" .)