Restricted mixer for gin in first class (6)
I believe the answer is:
finite
'restricted' is the definition.
(finite can describe a restricted or limited amount)
'mixer for gin in first class' is the wordplay.
'mixer for gin' becomes 'it' (Italian vermouth, as in a gin and it).
'in' means one lot of letters goes inside another.
'first class' becomes 'fine' (fine can mean fancy or first class).
'it' inserted into 'fine' is 'FINITE'.
(Other definitions for finite that I've seen before include "Having limits, not endless" , "Bounded or measurable" , "Having boundaries or limits" , "Bounded, having limits" , "Having a limit, not completely open-ended" .)