Brother to acknowledge prime minister (5)
I believe the answer is:
brown
'prime minister' is the definition.
(Gordon Brown, former UK PM)
'brother to acknowledge' is the wordplay.
'brother' becomes 'br' (abbreviation in titles of monks).
'to acknowledge' becomes 'own' (I've seen this in another clue).
'br'+'own'='BROWN'
(Other definitions for brown that I've seen before include "Capability" , "Colour of earth" , "Fry in a pan until it changes colour" , "bookish father" , "Prime Minister, 2007-2010" .)