Military unit cheers celebrity joining club (9)
I believe the answer is:
battalion
'military unit' is the definition.
(I know that battalion is a type of army unit)
'cheers celebrity joining club' is the wordplay.
'cheers' becomes 'ta' (both can mean thank you).
'celebrity' becomes 'lion' (both can mean a celebrated person).
'joining' is a charade indicator (letters next to each other).
'club' becomes 'bat' (as in sporting equipment).
'ta'+'lion'='talion'
'talion' put after 'bat' is 'BATTALION'.
(Other definitions for battalion that I've seen before include "Section of army" , "many troops" , "army group" , "Army unit" , "Army formation" .)