A three-man experiment? (5)
I believe the answer is:
trial
'experiment?' is the definition.
(trial is a kind of experiment)
'a three-man' is the wordplay.
'a three' becomes 'tri' (prefix meaning 'three').
'man' becomes 'al' ().
'tri'+'al'='TRIAL'
(Other definitions for trial that I've seen before include "Legal hearing" , "Troublesome thing; test" , "Legal proceedings or a test run" , "Pilot" , "Court proceedings" .)