One with special knowledge in back is returning red (7)
I believe the answer is:
insider
'one with special knowledge' is the definition.
(I've seen this before)
'in back is returning red' is the wordplay.
'back' is a reversal indicator.
'returning' shows that the letters should be reversed in order.
'is' written backwards gives 'si'.
'red' reversed gives 'der'.
'in'+'si'+'der'='INSIDER'
(Other definitions for insider that I've seen before include "one familiar with company dealings?" , "Illegal sort of dealing" , "one's not allowed to trade" , "one who's in the know" , "Accepted member of accepted group, has access to information" .)