Cast absent? So casual (9)
I believe the answer is:
throwaway
'casual' is the definition.
(I've seen this before)
'cast absent?' is the wordplay.
'cast' becomes 'throw' ('throw' can be a synonym of 'cast').
'absent?' becomes 'away' ('away' can be similar in meaning to 'absent').
'throw'+'away'='THROWAWAY'
'so' is the link.
(Other definitions for throwaway that I've seen before include "Disposable" , "Get rid of" , "Made to be discarded after one use" , "Intended to be discarded after use" , "(Of a remark) casual, understated" .)