Viewer sees lord outside gym (6)
I believe the answer is:
peeper
'viewer' is the definition.
(peeper is a kind of viewer)
'lord outside gym' is the wordplay.
'lord' becomes 'peer' (peer is a kind of lord).
'outside' means one lot of letters goes inside another.
'gym' becomes 'pe'.
'peer' enclosing 'pe' is 'PEEPER'.
'sees' is the link.
(Other definitions for peeper that I've seen before include "Eye (slang)" , "Voyeur" , "Viewer" .)