Fix a drink infused with pop mostly (7)
I believe the answer is:
appoint
'fix' is the definition.
(I've seen this before)
'a drink infused with pop mostly' is the wordplay.
'drink' becomes 'pint' (pint is a unit of liquid volume).
'infused with' means one lot of letters goes inside another.
'mostly' means to remove the last letter (most but not all of the word).
'pop' with its last letter taken away is 'po'.
'pint' going around 'po' is 'ppoint'.
'a'+'ppoint'='APPOINT'
(Other definitions for appoint that I've seen before include "Assign job to (someone)" , "Ordain" , "get the post sorted" , "Designate (job)" , "Fix" .)