Forward a letter for a person (3)
I believe the answer is:
one
'person' is the definition.
('one' can mean an unspecified person)
'forward a letter' is the wordplay.
'forward' becomes 'on'.
'a letter' becomes 'e' ().
'on'+'e'='ONE'
'for a' is the link.
(Other definitions for one that I've seen before include "A certain; single person" , "time" , "SECOND PART OF 16" , "Identical; number" , "joke" .)