Not planned, a topless western! (10)
I believe the answer is:
accidental
'not planned' is the definition.
(I know this)
'a topless western' is the wordplay.
'topless' means to remove the first letter.
'western' becomes 'occidental' (similar in meaning).
'occidental' with its first letter taken off is 'ccidental'.
'a'+'ccidental'='ACCIDENTAL'
(Other definitions for accidental that I've seen before include "Happening by chance" , "Sign raising or lowering a musical note" , "Unintended - sort of note" , "Possible sharp" , "change of key" .)