Show retired brother familiar saying (7)
I believe the answer is:
proverb
'familiar saying' is the definition.
(I've seen this before)
'show retired brother' is the wordplay.
'show' becomes 'prove' (proving is a kind of showing).
'retired' shows that the letters should be reversed in order.
'brother' becomes 'br'.
'br' in reverse letter order is 'rb'.
'prove'+'rb'='PROVERB'
(Other definitions for proverb that I've seen before include "Old saw" , "Aphorism" , "Generally true saying" , "OT saying" , "Familiar saying" .)