Impulse to dump gallons in river (4)
I believe the answer is:
urge
'impulse' is the definition.
(synonyms)
'gallons in river' is the wordplay.
'gallons' becomes 'g' (abbreviation).
'in' means one lot of letters goes inside another.
'river' becomes 'ure' (river in Yorkshire).
'g' going into 'ure' is 'URGE'.
'to dump' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for urge that I've seen before include "Encourage (to do something)" , "Encourage to proceed, drive onwards" , "Advise earnestly, prod" , "Advocate" , "Itch" .)