Four outsiders in theatre with unfinished business (6)
I believe the answer is:
tetrad
'four' is the definition.
('tetrad' can be a synonym of 'four')
'outsiders in theatre with unfinished business' is the wordplay.
'outsiders in' suggests removing the centre.
'with' means one lot of letters go next to another.
'unfinished' means to remove the last letter.
'business' becomes 'trade' (trade is a kind of business).
'theatre' with its centre taken out is 'te'.
'trade' with its last letter taken away is 'trad'.
'te'+'trad'='TETRAD'
(Other definitions for tetrad that I've seen before include "Group or set of four" , "number of Gospel writers" , "quartet" .)