Leg in cast -- chap being careless (9)
I believe the answer is:
negligent
'careless' is the definition.
(similar in meaning)
'leg in cast chap' is the wordplay.
'cast' indicates an anagram (letters are dispersed or cast around).
'chap' becomes 'gent' (I've seen this in other clues).
'leg'+'in'='legin'
'legin' with letters rearranged gives 'negli'.
'negli'+'gent'='NEGLIGENT'
'being' acts as a link.
(Other definitions for negligent that I've seen before include "Of the study of insects" , "slack" , "Not minding" , "Failing to take proper care" , "Careless of duties" .)