Cook fish maybe in Gulf Stream (5)
I believe the answer is:
grill
'cook fish maybe' is the definition.
(fish can be grilled)
'gulf stream' is the wordplay.
'gulf' becomes 'G' (geographical abbreviation).
'stream' becomes 'rill' (rill is a kind of stream).
'g'+'rill'='GRILL'
'in' acts as a link.
(Other definitions for grill that I've seen before include "Cook under a flame" , "Seriously question" , "Framework of metal bars" , "Means of cooking" , "pump" .)