What secret agent might use to see schooner? (8)
I believe the answer is:
spyglass
'what secret agent might use' is the definition.
(a spy might look at something with a spyglass)
'see schooner?' is the wordplay.
'see' becomes 'spy' (spy can mean to see or observe).
'schooner?' becomes 'glass' (schooner is a type of glass).
'spy'+'glass'='SPYGLASS'
'to' is the link.
(Other definitions for spyglass that I've seen before include "that magnifies" , "he can see through it!" , "Viewing aid" , "little scope" , "Aid to pirate, perhaps" .)