Short-lived naval force in entrance to Gulf (8)
I believe the answer is:
fleeting
'short-lived' is the definition.
(thesaurus)
'naval force in entrance to gulf' is the wordplay.
'naval force' becomes 'fleet' (I've seen this before).
'entrance to' suggests taking the first letters.
The first letter of 'gulf' is 'g'.
'fleet'+'in'+'g'='FLEETING'
(Other definitions for fleeting that I've seen before include "Ephemeral" , "Transient" , "Evanescent" , "fast" , "Lasting for a brief time" .)