Telescope for a secret agent on a vessel (8)
I believe the answer is:
spyglass
'telescope' is the definition.
(I know that spyglass is a type of refracting telescope)
'a secret agent on a vessel' is the wordplay.
'a secret agent' becomes 'spy' (spy is a kind of secret agent).
'on a vessel' becomes 'glass' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'spy'+'glass'='SPYGLASS'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for spyglass that I've seen before include "that magnifies" , "viewer" , "Small refracting telescope" , "little scope" , "he can see through it!" .)