Chap gets something for driving off a water mammal (7)
I believe the answer is:
manatee
'mammal' is the definition.
(manatee is a kind of mammal)
'chap gets something' is the wordplay.
'chap' becomes 'man' (chap can mean a man).
'gets' is a charade indicator (letters next to each other).
'something' becomes 'atee' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'man'+'atee'='MANATEE'
'for driving off a water' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for manatee that I've seen before include "Sea cow - emanate (anag)" , "sea creature appears" , "Sea denizen" , "Large swimmer" , "Beast" .)