Greek character behind catalogue is a collector (11)
I believe the answer is:
philatelist
'collector' is the definition.
(I know that philatelist is a type of collector)
'greek character behind catalogue' is the wordplay.
'greek character' becomes 'phi' (letter from the Greek alphabet).
'behind' becomes 'late' (as in behind schedule).
'catalogue' becomes 'list' (a catalogue is a list of items).
'phi'+'late'+'list'='PHILATELIST'
'is a' acts as a link.
(Other definitions for philatelist that I've seen before include "Stamp collector" , "Person with hobby" .)