Retiring manner adopted by French mendicant (5)
I believe the answer is:
friar
'mendicant' is the definition.
('friar' can be a synonym of 'mendicant')
'retiring manner adopted by french' is the wordplay.
'retiring' shows that the letters should be reversed in order.
'manner' becomes 'air' (air is a kind of manner).
'adopted by' means one lot of letters goes inside another (I've seen this in other clues).
'french' becomes 'fr'.
'air' backwards is 'ria'.
'ria' placed into 'fr' is 'FRIAR'.
(Other definitions for friar that I've seen before include "Father" , "Mendicant priest" , "Eg, a Franciscan" , "Religious man" , "Member of one of the four mendicant orders" .)