Club thanks cat regiment (9)
I believe the answer is:
battalion
'regiment' is the definition.
(thesaurus)
'club thanks cat' is the wordplay.
'club' becomes 'bat' (as in sporting equipment).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'cat' becomes 'lion' (lion is a kind of cat).
'bat'+'ta'+'lion'='BATTALION'
(Other definitions for battalion that I've seen before include "Large body of troops" , "Military unit" , "Body of soldiers" , "army group" , "group of companies" .)