Drive out leader of poachers in river and lake (5)
I believe the answer is:
expel
'drive out' is the definition.
(I've seen this in another clue)
'leader of poachers in river and lake' is the wordplay.
'leader of' suggests taking the first letters.
'in' is an insertion indicator.
'river' becomes 'exe' (River Exe).
'and' is a charade indicator (letters next to each other).
'lake' becomes 'L' (geographical abbreviation).
The first letter of 'poachers' is 'p'.
'p' put inside 'exe' is 'expe'.
'expe'+'l'='EXPEL'
(Other definitions for expel that I've seen before include "Drive out - dispossess" , "Throw out" , "bar" , "Discharge in disgrace" , "Drive out, banish" .)