They rigorously investigate every deal leaders attempted (5)
I believe the answer is:
tried
'attempted' is the definition.
('try' can be a synonym of 'attempt')
'they rigorously investigate every deal leaders' is the wordplay.
'leaders' suggests taking the first letters.
The initial letters of 'they rigorously investigate every deal' is 'TRIED'.
(Other definitions for tried that I've seen before include "Put to the test" , "Proven by experience" , "Essayed" , "Had a crack" , "tasted" .)