Examine source of Gulf Stream (5)
I believe the answer is:
grill
'examine' is the definition.
(I know that grill is a more specific form of the action examine)
'source of gulf stream' is the wordplay.
'source of' suggests taking the first letters (the start - or source - of the word).
'stream' becomes 'rill' (rill is a kind of stream).
The initial letter of 'gulf' is 'g'.
'g'+'rill'='GRILL'
(Other definitions for grill that I've seen before include "Question hard" , "Question closely" , "Question aggressively" , "Cook under direct heat" , "Cook under a flame" .)