Best of British beer good after siesta (5,1,3)
I believe the answer is:
break a leg
'best of british' is the definition.
(both can mean good luck)
'beer good after siesta' is the wordplay.
'beer' becomes 'ale' (type of beer).
'good' becomes 'g' (abbreviation).
'after' is a charade indicator (letters next to each other).
'siesta' becomes 'break' (a siesta is a break or sleep).
'ale'+'g'='aleg'
'aleg' put after 'break' is 'BREAK A LEG'.
(Other definitions for break a leg that I've seen before include "Actors' good luck" , "Good luck wish offered to actor in bleak rage" , "Good luck in the theatre!" , "leading to member being in cast?" , "Do something that requires surgery" .)