Be joining teams in addition (7)
I believe the answer is:
besides
'in addition' is the definition.
(I've seen this in another clue)
'be joining teams' is the wordplay.
'joining' says to put letters next to each other.
'teams' becomes 'sides' (eg football sides).
'be'+'sides'='BESIDES'
(Other definitions for besides that I've seen before include "Anyway" , "to boot" , "In addition, as well" , "See bids (anag)" , "Apart from" .)