Extra: a run made without referring to the score (2,3)
I believe the answer is:
by ear
'made without referring to the score' is the definition.
'extra a run' is the wordplay.
'extra' becomes 'bye' (cricket term).
'a run' becomes 'ar' (I've seen this in other clues).
'bye'+'ar'='BY EAR'
(Other definitions for by ear that I've seen before include "Of music played without referring to a written score" , "without reading from written music" .)