Spy initially navigates rescue boat (4)
I believe the answer is:
nark
'spy' is the definition.
(I've seen this before)
'initially navigates rescue boat' is the wordplay.
'initially' says to take the initial letters.
'rescue boat' becomes 'ark' (rescue boat during the biblical flood).
The first letter of 'navigates' is 'n'.
'n'+'ark'='NARK'
(Other definitions for nark that I've seen before include "Annoy; police informer" , "one causing annoyance down under" , "cause irritation" , "Get to" , "Cause minor annoyance" .)