Retiring manner adopted by father's brother (5)
I believe the answer is:
friar
'brother' is the definition.
(thesaurus)
'retiring manner adopted by father's' is the wordplay.
'retiring' says the letters should be written backwards.
'manner' becomes 'air' (air is a kind of manner).
'adopted by' indicates putting letters inside (I've seen this in other clues).
'father' becomes 'fr'.
'air' reversed gives 'ria'.
'ria' inserted inside 'fr' is 'FRIAR'.
(Other definitions for friar that I've seen before include "Eg, a Dominican" , "Mendicant priest" , "Father" , "Male member of a mendicant religious order" , "Monk perhaps" .)