Say 51 expelled by monks? Shockingly bad! (9)
I believe the answer is:
egregious
'shockingly bad' is the definition.
'say 51 expelled by monks?' is the wordplay.
'say' becomes 'eg'.
'51' becomes 'LI' (roman numeral).
'expelled by' suggests deleting specific letters.
'monks?' becomes 'religious' (monks are an example of religious people).
'religious' with 'li' taken out is 'regious'.
'eg'+'regious'='EGREGIOUS'
(Other definitions for egregious that I've seen before include "Notorious" , "Insufferable" , "Extraordinary" , "Outstandingly bad" , "Shocking" .)