Hold forth, initially dismissing extravagant allegation (7)
I believe the answer is:
declaim
'hold forth' is the definition.
(I've seen this before)
'initially dismissing extravagant allegation' is the wordplay.
'initially' says to take the initial letters.
'allegation' becomes 'claim' (I've seen this before).
The initial letters of 'dismissing extravagant' is 'de'.
'de'+'claim'='DECLAIM'
(Other definitions for declaim that I've seen before include "Make a rhetorical speech" , "How orator will" , "Announce" , "Hold forth" , "Make a set speech" .)