Unusually bad scoreline is of some importance (12)
I believe the answer is:
considerable
'of some importance' is the definition.
(I've seen this before)
'unusually bad scoreline' is the wordplay.
'unusually' indicates anagramming the letters.
'bad'+'scoreline'='badscoreline'
'badscoreline' is an anagram of 'CONSIDERABLE'.
'is' is the link.
(Other definitions for considerable that I've seen before include "Major" , "Relatively large" , "Significant" , "worth entertaining?" , "Fairly large or numerous" .)