One poor black moth (6)
I believe the answer is:
beggar
'one poor' is the definition.
(a poor person)
'black moth' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'moth' becomes 'eggar' (eggar is a kind of moth).
'b'+'eggar'='BEGGAR'
(Other definitions for beggar that I've seen before include "Person who can't choose?" , "Pauper or supplicant" , "Pauper who seeks alms" , "One may entreat you" , "bum" .)