Run, when in better shape, at first without a stitch (8)
I believe the answer is:
starkers
'without a stitch' is the definition.
(both can mean naked)
'run when in better shape at first' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'when in' is an insertion indicator.
'better' becomes 'staker' (someone betting a stake).
'at first' indicates taking the first letters.
The initial letter of 'shape' is 's'.
'r' put within 'staker' is 'starker'.
'starker'+'s'='STARKERS'
(Other definitions for starkers that I've seen before include "uncovered" , "Slangily naked" , "Slangily nude - starts being stellar" , "in his birthday suit?" , "Wearing no clothes" .)