Drink gallon where plastered member might be? (3,5)
I believe the answer is:
gin sling
'drink' is the definition.
(gin sling is a kind of drink)
'gallon where plastered member might be?' is the wordplay.
'gallon' becomes 'g' (abbreviation for the unit of liquid).
'where plastered member might be?' becomes 'insling' (an arm in plaster may be in a sling).
'g'+'insling'='GIN SLING'
(Other definitions for gin sling that I've seen before include "alcoholic drink" , "Spirit flavoured with juniper berries to which lemon or lime juice is added" , "Iced spirit and water sweetened with lemon or lime juice" .)