Posers for supply teacher (9)
I believe the answer is:
professor
'teacher' is the definition.
(thesaurus)
'posers for supply' is the wordplay.
'supply' indicates anagramming the letters (in a supple way).
'posers'+'for'='posersfor'
'posersfor' anagrammed gives 'PROFESSOR'.
(Other definitions for professor that I've seen before include "Senior university teacher" , "chairman?" , "Fellow" , "One affirming faith" , "High-ranking academic" .)