Brother -- that is cunning, in short (7)
I believe the answer is:
briefly
'in short' is the definition.
(synonyms)
'brother that is cunning' is the wordplay.
'brother' becomes 'br' (abbreviation in titles of monks).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'cunning' becomes 'fly' (fly can mean skilful or sharp).
'br'+'ie'+'fly'='BRIEFLY'
(Other definitions for briefly that I've seen before include "Concisely" , "For a short time" , "In a nutshell" , "Fleetingly" , "Cursorily" .)