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 before)
'leader of poachers in river and lake' is the wordplay.
'leader of' says to take the initial 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' placed 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" .)