One short of a score regularly unfit, even after practice (8)
I believe the answer is:
nineteen
'one short of a score' is the definition.
(score can mean twenty)
'regularly unfit even after practice' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'even' becomes 'een' (old-fashioned contraction).
'after' means one lot of letters go next to another.
'practice' becomes 'net' (net can mean cricket practice).
The alternating letters of 'unfit' are 'ni'.
'een' after 'net' is 'neteen'.
'ni'+'neteen'='NINETEEN'
(Other definitions for nineteen that I've seen before include "Highest teenage age" , "No. talked to the dozen" , "Upper age limit for 2 Down" , "Number (at the clubhouse?)" , "Roman XIX" .)