Fair exchange, registered as a monk (5)
I believe the answer is:
friar
'a monk' is the definition.
(friar is a type of monk)
'fair exchange registered' is the wordplay.
'exchange' indicates an anagram.
'registered' becomes 'R' (abbreviation, as in registered trademark).
'fair' is an anagram of 'fria'.
'fria'+'r'='FRIAR'
'as' is the link.
(Other definitions for friar that I've seen before include "Male member of a mendicant religious order" , "Monk perhaps" , "Mendicant monk" , "Man connected with order" , "Eg, a Dominican" .)