Discharge from canal found in river and mouth of estuary (7)
I believe the answer is:
execute
'discharge' is the definition.
(I've seen this before)
'canal found in river and mouth of estuary' is the wordplay.
'canal' becomes 'cut' (cut is a kind of canal).
'found in' indicates putting letters inside.
'river' becomes 'exe' (River Exe).
'and' says to put letters next to each other.
'mouth of' says to take the initial letters (mouth can mean something's origin).
The initial letter of 'estuary' is 'e'.
'exe'+'e'='exee'
'cut' put inside 'exee' is 'EXECUTE'.
'from' acts as a link.
(Other definitions for execute that I've seen before include "run a program" , "Carry out or kill legally" , "ice" , "Put (plan) into effect; kill (a condemned person)" , "Guillotine" .)