Reg carries six back for the donor (5)
I believe the answer is:
giver
'the donor' is the definition.
(synonyms)
'reg carries six back' is the wordplay.
'carries' means one lot of letters goes inside another (inserted letters carried inside).
'six' becomes 'VI' (Roman numerals).
'back' is a reversal indicator.
'reg' going around 'vi' is 'revig'.
'revig' back-to-front is 'GIVER'.
'for' is the link.
(Other definitions for giver that I've seen before include "Donor" , "Present source" , "Bestower" , "presenter" .)