Telescope for secret agent on schooner, perhaps (8)
I believe the answer is:
spyglass
'telescope' is the definition.
(spyglass is a kind of telescope)
'secret agent on schooner perhaps' is the wordplay.
'secret agent' becomes 'spy' (spy is a kind of secret agent).
'on' says to put letters next to each other.
'schooner perhaps' becomes 'glass' (schooner is a kind of glass).
'spy'+'glass'='SPYGLASS'
'for' is the link.
(Other definitions for spyglass that I've seen before include "Viewing aid" , "little scope" , "Small refracting telescope" , "that magnifies" , "he can see through it!" .)