Marine killer otherwise about ... (4)
I believe the answer is:
orca
'marine killer' is the definition.
(I've seen this in another clue)
'otherwise about' is the wordplay.
'otherwise' becomes 'or'.
'about' becomes 'ca' (short for circa).
'or'+'ca'='ORCA'
(Other definitions for orca that I've seen before include "Large mammal" , "Killer at sea" , "Marine creature" , "27 [WHALE]" , "Sea creature" .)