Drunk on just under two pints (5)
I believe the answer is:
litre
'just under two pints' is the definition.
(a litre is less than two pints)
'drunk on' is the wordplay.
'drunk' becomes 'lit' (I've seen this in another clue).
'on' becomes 're' ('on' can be mean 'with reference to').
'lit'+'re'='LITRE'
(Other definitions for litre that I've seen before include "Mcc?" , "less than two pints" , "1.7598 UK pints" , "One thousand cubic centimetres" , "Nearly two pints" .)