Cast here for Equus taking part accordingly (9)
I believe the answer is:
therefore
'accordingly' is the definition.
(thesaurus)
'cast here for equus taking part' is the wordplay.
'taking part' indicates the answer is hidden within the clue.
'THEREFORE' is hidden within 'cast here for equus'.
(Other definitions for therefore that I've seen before include "Ergo" , "Consequently - for that reason" , "as result" , "It follows" , "In consequence" .)