Old British minister to spy (7)
I believe the answer is:
observe
'to spy' is the definition.
(I know that observe is a more specific form of the action sight)
'old british minister' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'british' becomes 'b' (abbreviation e.g. in BBC).
'minister' becomes 'serve' (I've seen this before).
'o'+'b'+'serve'='OBSERVE'
(Other definitions for observe that I've seen before include "Mark" , "Keep an eye on" , "Notice, perceive" , "taking notice" , "Obverse (anag.)" .)