Stream bound to hold gallons (5)
I believe the answer is:
surge
'stream' is the definition.
(thesaurus)
'bound to hold gallons' is the wordplay.
'bound to' becomes 'sure' (synonyms).
'hold' is an insertion indicator.
'gallons' becomes 'G' (abbreviation).
'sure' placed around 'g' is 'SURGE'.
(Other definitions for surge that I've seen before include "Sudden increase, gush" , "Sudden influx" , "Movement in crowd" , "Move rapidly forward like a wave" , "Powerful rush (of emotion)" .)