Strange thing that is worn in retirement? (7)
I believe the answer is:
nightie
'worn in retirement?' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'strange thing that is' is the wordplay.
'strange' is an anagram indicator.
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'thing' is an anagram of 'night'.
'night'+'ie'='NIGHTIE'
(Other definitions for nightie that I've seen before include "Coverage for the rest" , "Skimpy attire" , "Lingerie item" , "cover for retiring" , "Garment for ladies" .)