Nothing cool for retirement? Chill naked (5)
I believe the answer is:
nihil
'nothing' is the definition.
('nihil' can be a synonym of 'nothing')
'cool for retirement? chill naked' is the wordplay.
'cool' becomes 'in' (I've seen this in other clues).
'for retirement?' says the letters should be written in reverse (retire can mean to retreat or go backwards).
'naked' indicates the central letters.
The middle letters of 'chill' are 'hil'.
'in' back-to-front is 'ni'.
'ni'+'hil'='NIHIL'
(Other definitions for nihil that I've seen before include "love" , "'... obstat' - Catholic imprimatur or approval" , "Nothing (Latin)" .)