Score unbelievable by Yorkshire's opener (5)
I believe the answer is:
tally
'score' is the definition.
(tally is a kind of score)
'unbelievable by yorkshire's opener' is the wordplay.
'unbelievable' becomes 'tall' (e.g. 'a tall story').
'by' is a charade indicator (letters next to each other).
'opener' suggests taking the first letters.
The initial letter of 'yorkshire' is 'y'.
'tall'+'y'='TALLY'
(Other definitions for tally that I've seen before include "List" , "Record; current score" , "Reckon up" , "Match - score" , "label" .)