Swell certainly to consume gallons (5)
I believe the answer is:
surge
'swell' is the definition.
(I've seen this before)
'certainly to consume gallons' is the wordplay.
'certainly' becomes 'sure' ('sure' can be a synonym of 'certainly').
'to consume' is an insertion indicator.
'gallons' becomes 'g'.
'sure' placed around 'g' is 'SURGE'.
(Other definitions for surge that I've seen before include "roll" , "Movement in crowd" , "Sudden rapid rise" , "Increase rapidly in intensity" , "Jerk" .)