Discharge pollution initially in river and lake (5)
I believe the answer is:
expel
'discharge' is the definition.
(I know that discharge can be written as expel)
'pollution initially in river and lake' is the wordplay.
'initially' indicates taking the first letters.
'in' is an insertion indicator.
'river' becomes 'exe' (River Exe).
'and' means one lot of letters go next to another.
'lake' becomes 'L' (geographical abbreviation).
The initial letter of 'pollution' is 'p'.
'p' going within 'exe' is 'expe'.
'expe'+'l'='EXPEL'
(Other definitions for expel that I've seen before include "Oust" , "Remove from membership" , "Throw out" , "Drive out, banish" , "Drive out with force" .)