Old boy's always purchasing 'right' Spectator (8)
I believe the answer is:
observer
'spectator' is the definition.
(thesaurus)
'old boy's always purchasing right' is the wordplay.
'old boy's' becomes 'obs' (I've seen this in other clues).
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'purchasing' indicates putting letters inside (some letters take possession of others).
'right' becomes 'r' (common abbreviation).
'ever' enclosing 'r' is 'erver'.
'obs'+'erver'='OBSERVER'
(Other definitions for observer that I've seen before include "He sees" , "Spectator - or another weekly publication!" , "Someone watching" , "One keeping his eye on" , "Sunday newspaper" .)