Down perhaps, reckon needing holiday finally (6)
I believe the answer is:
county
'down perhaps' is the definition.
Although both the answer and definition are adjectives, I don't see how one could define the other.
'reckon needing holiday finally' is the wordplay.
'reckon' becomes 'count' (I've seen this before).
'needing holiday finally' becomes 'y' (I can't justify this - if you can you should believe this answer much more).
'count'+'y'='COUNTY'
Can you help me to learn more?
(Other definitions for county that I've seen before include "Unit into which country is divided for local government" , "Territorial division" , "Eg, Dorset" , "Shire" , "Down for one" .)