Catalogue containing old book by political operator (8)
I believe the answer is:
lobbyist
'political operator' is the definition.
(someone who lobbies politicians)
'catalogue containing old book by' is the wordplay.
'catalogue' becomes 'list' (a catalogue is a list of items).
'containing' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'book' becomes 'b' (abbreviation for book).
'o'+'b'+'by'='obby'
'list' enclosing 'obby' is 'LOBBYIST'.
(Other definitions for lobbyist that I've seen before include "pusher" , "'To - it may concern'" , "One seeks to influence legislators" , "figure influencing lawmakers" , "political campaigner" .)