Cast with a manner that’s casual (9)
I believe the answer is:
throwaway
'casual' is the definition.
(I've seen this before)
'cast with a manner' is the wordplay.
'cast' becomes 'throw' (synonyms).
'with' means one lot of letters go next to another.
'manner' becomes 'way' (synonyms).
'throw'+'a'+'way'='THROWAWAY'
'that's' acts as a link.
(Other definitions for throwaway that I've seen before include "Disposable" , "Intended to be discarded after use" , "Get rid of" , "This remark is deliberately offhand - cast it aside" , "Made to be discarded after one use" .)