Therefore catalogue must include old performer (7)
I believe the answer is:
soloist
'performer' is the definition.
(I have seen 'Lone performer' mean 'soloist' so perhaps 'performer' could also mean 'soloist')
'therefore catalogue must include old' is the wordplay.
'therefore' becomes 'so'.
'catalogue' becomes 'list' (I've seen this before).
'must include' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'list' placed around 'o' is 'loist'.
'so'+'loist'='SOLOIST'
(Other definitions for soloist that I've seen before include "Virtuoso player" , "Individual performer" , "Lone performer" , "musician at front of stage?" , "One who performs" .)