Sightseer: gentleman about to be taken in by spiv? (7)
I believe the answer is:
tourist
'sightseer' is the definition.
(someone visiting certain sights)
'gentleman about to be taken in by spiv?' is the wordplay.
'gentleman' becomes 'sir' (a man may be addressed as sir).
'about' is a reversal indicator.
'to be taken in by' means one lot of letters goes inside another.
'spiv?' becomes 'tout' (both can mean a disreputable salesman).
'sir' reversed gives 'ris'.
'ris' inserted into 'tout' is 'TOURIST'.
(Other definitions for tourist that I've seen before include "Tripper, sightseer" , "Traveller for pleasure" , "Visiting sightseer" , "Holiday visitor" , "Sightseeing traveller" .)