Agree to clubs participating in expert training (6)
I believe the answer is:
accept
'agree to' is the definition.
(thesaurus)
'clubs participating in expert training' is the wordplay.
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'participating in' indicates putting letters inside.
'expert' becomes 'ace' (ace is a kind of expert**).
'training' becomes 'pt'.
'c' going inside 'ace' is 'acce'.
'acce'+'pt'='ACCEPT'
(Other definitions for accept that I've seen before include "Receive willingly or take as true" , "agree to buy" , "Put up with" , "Take on; tolerate" , "Receive the gift or agree to comply" .)