Erin lad's possibly is one? (8)
I believe the answer is:
islander
'one?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'erin lad's possibly' is the wordplay.
'possibly' is an anagram indicator.
'erin'+'lads'='erinlads'
'erinlads' is an anagram of 'ISLANDER'.
'is' acts as a link.
(Other definitions for islander that I've seen before include "Inhabitant of Man, for example" , "Eg, Haitian or Cretan" , "Samoan, maybe" , "One living away from mainland" , "One living surrounded by water" .)