Only a little drink, but Joe left light-headed (4)
I believe the answer is:
gill
'only a little drink' is the definition.
I can't tell whether this defines the answer.
'joe left light-headed' is the wordplay.
'joe' becomes 'GI' (GI Joe).
'left light-headed' becomes 'll' (this could be a standard abbreviation I've not previously seen).
'gi'+'ll'='GILL'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for gill that I've seen before include "Organ for breathing under water" , "Fish's breathing apparatus" , "Fish's breathing organ" , "One quarter of a pint" , "Eric ---, sculptor; breathing organ" .)