Mix gallons in pool (5)
I believe the answer is:
merge
'mix' is the definition.
(I know that mix can be written as merge)
'gallons in pool' is the wordplay.
'gallons' becomes 'G' (abbreviation).
'in' indicates putting letters inside.
'pool' becomes 'mere' (mere can mean a lake).
'g' inserted 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" .)