Trades lead forms on Sunday (5)
I believe the answer is:
deals
'trades' is the definition.
('deal' can be a synonym of 'trade')
'lead forms on sunday' is the wordplay.
'forms' indicates anagramming the letters.
'on' is a charade indicator (letters next to each other).
'sunday' becomes 's' (**).
'lead' with letters rearranged gives 'deal'.
'deal'+'s'='DEALS'
(Other definitions for deals that I've seen before include "Passes cards around" , "Trades" , "Bargains" , "Business transactions" , "Business agreements" .)