Do but cast suspicion (5)
I believe the answer is:
doubt
'suspicion' is the definition.
(thesaurus)
'do but cast' is the wordplay.
'cast' indicates anagramming the letters.
'but' anagrammed gives 'ubt'.
'do'+'ubt'='DOUBT'
(Other definitions for doubt that I've seen before include "Distrust" , "Suspect" , "One certainly can't have it" , "Feel uncertain about" , "Be uncertain about or consider unlikely" .)