One may be under fire, seeing price put on gallon (5)
I believe the answer is:
grate
'one may be under fire' is the definition.
'price put on gallon' is the wordplay.
'price' becomes 'rate' (I've seen this in other clues).
'put on' means one lot of letters go next to another.
'gallon' becomes 'g' (abbreviation for the unit of liquid).
'rate' after 'g' is 'GRATE'.
'seeing' is the link.
(Other definitions for grate that I've seen before include "Shred - jar" , "Make a harsh rasping sound" , "Rasp; fire holder" , "Great (anag.)" , "in which Santa lands?" .)