Great wave bound to contain gallons (5)
I believe the answer is:
surge
'great wave' is the definition.
(I've seen this before)
'bound to contain gallons' is the wordplay.
'bound to' becomes 'sure' (synonyms).
'contain' means one lot of letters goes inside another.
'gallons' becomes 'g' (abbreviation).
'sure' enclosing 'g' is 'SURGE'.
(Other definitions for surge that I've seen before include "Increase rapidly in intensity" , "Sudden onward motion" , "significant increase" , "Sudden rapid rise" , "Sudden push forward" .)