First seen in Nightingale, undying resilience supporting each CO (5)
I believe the answer is:
nurse
'co' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'first seen in nightingale undying resilience supporting each' is the wordplay.
'first seen in' says to take the initial letters.
The first letters of 'nightingale undying resilience supporting each' is 'NURSE'.
(Other definitions for nurse that I've seen before include "Medical carer" , "Care for - harbour" , "Trained carer" , "Qualified carer" , "Nanny" .)