Third man is unusually able (4)
I believe the answer is:
abel
'third man' is the definition.
(I've seen this before)
'unusually able' is the wordplay.
'unusually' indicates an anagram.
'able' with letters rearranged gives 'ABEL'.
'is' acts as a link.
(Other definitions for abel that I've seen before include "Cornish city" , "Son of Adam and Eve" , "Adamson" , "Second child" , "Brother killed by Cain" .)