Agree expert is concealing three consecutive letters (6)
I believe the answer is:
accede
'agree' is the definition.
(acceding is a kind of agreeing)
'expert is concealing three consecutive letters' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'is concealing' is an insertion indicator.
'three consecutive letters' becomes 'CED' (CDE are consecutive letters).
'ace' going around 'ced' is 'ACCEDE'.
(Other definitions for accede that I've seen before include "Consent - agree" , "bow" , "Assent (to something)" , "'Acquiesce, defer (6)'" , "Grant or consent" .)