Endorse Anglo-French compact (6)
I believe the answer is:
affirm
'endorse' is the definition.
(thesaurus)
'anglo-french compact' is the wordplay.
'anglo-french' becomes 'af' (abbreviation in linguistics).
'compact' becomes 'firm' ('firm' can be similar in meaning to 'compact'**).
'af'+'firm'='AFFIRM'
(Other definitions for affirm that I've seen before include "Declare, assert" , "Verify" , "Avow or corroborate" , "Asseverate" , "Emphatically state" .)