Begin unusual run as top of the bill (4,4)
I believe the answer is:
star turn
'top of the bill' is the definition.
(I've seen this before)
'begin unusual run' is the wordplay.
'begin' becomes 'start' (synonyms).
'unusual' indicates an anagram.
'run' is an anagram of 'urn'.
'start'+'urn'='STAR-TURN'
'as' acts as a link.
(Other definitions for star turn that I've seen before include "Most impressive act" , "fine performance" , "Top of the bill" , "Most heralded act" , "Top act" .)