Spot secret agent in jetty (4)
I believe the answer is:
mole
I believe this is a triple definition!
'spot' is the first definition.
(I've seen this before)
'secret agent' is the second definition.
(mole is a kind of secret agent)
'in' acts as a link.
'jetty' is the third definition.
(synonyms)
(Other definitions for mole that I've seen before include "Pier" , "Subterranean mammal" , ""Gentleman in black velvet''" , "Counterspy" , "Jetty; animal" .)