Mix gallons in pool (5)
I believe the answer is:
merge
'mix' is the definition.
(synonyms)
'gallons in pool' is the wordplay.
'gallons' becomes 'G' (abbreviation).
'in' is an insertion indicator.
'pool' becomes 'mere' (mere can mean a lake).
'g' put into 'mere' is 'MERGE'.
(Other definitions for merge that I've seen before include "Combine into one" , "Make one" , "Blend, coalesce" , "Combine, coalesce" , "Become one" .)