Agree with 100 performing well (7)
I believe the answer is:
conform
'agree' is the definition.
(conforming is a kind of agreeing)
'with 100 performing well' is the wordplay.
'with' means one lot of letters go next to another.
'100' becomes 'C' (Roman numeral).
'performing well' becomes 'on form' (someone on form is performing at a good standard).
'c'+'onform' is 'CONFORM'.
(Other definitions for conform that I've seen before include "Comply with accepted rules" , "Fall into line" , "Agree" , "Follow the herd" , "Match" .)