Discharge source of pollution into river and lake (5)
I believe the answer is:
expel
'discharge' is the definition.
('expel' can be a synonym of 'discharge')
'source of pollution into river and lake' is the wordplay.
'source of' suggests taking the first letters.
'into' is an insertion indicator.
'river' becomes 'exe' (I've seen this before**).
'and' means one lot of letters go next to another.
'lake' becomes 'l'.
The first letter of 'pollution' is 'p'.
'p' put into 'exe' is 'expe'.
'expe'+'l'='EXPEL'
(Other definitions for expel that I've seen before include "Kick out" , "Drive away" , "Evict" , "Boot out" , "Remove from membership" .)