Blend gallons into lake (5)
I believe the answer is:
merge
'blend' is the definition.
(I know that blend can be written as merge)
'gallons into lake' is the wordplay.
'gallons' becomes 'G' (abbreviation).
'into' is an insertion indicator.
'lake' becomes 'mere' (mere is a kind of lake).
'g' inserted within 'mere' is 'MERGE'.
(Other definitions for merge that I've seen before include "Combine to form a single entity" , "Combine elements" , "Come together as one" , "Make one" , "Become combined" .)