I, perhaps, mourn real man failing (5,7)
I believe the answer is:
roman numeral
'i' is the definition.
(I is the Roman numeral for one)
'mourn real man failing' is the wordplay.
'failing' is an anagram indicator (a failed spelling of the word).
'mourn'+'real'+'man'='mournrealman'
'mournrealman' with letters rearranged gives 'ROMAN NUMERAL'.
'perhaps' is the link.
(Other definitions for roman numeral that I've seen before include "Such as V, X or L" , "I, perhaps" , "Eg, X or V or M" , "I for one!" , "I say" .)