Thus behold a lead break (4)
I believe the answer is:
solo
'a lead break' is the definition.
(break in a song taken by a lead musician)
'thus behold' is the wordplay.
'thus' becomes 'so' (synonyms).
'behold' becomes 'lo' (archaic exclamation meaning 'look!').
'so'+'lo'='SOLO'
(Other definitions for solo that I've seen before include "Type of whist" , "Unaccompanied - card game" , "By oneself" , "Lone" , "Unaccompanied passage" .)