Absurd thing, that is, to wear (7)
I believe the answer is:
nightie
'to wear' is the definition.
(nightie is a kind of wear)
'absurd thing that is' is the wordplay.
'absurd' is an anagram indicator.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'thing' is an anagram of 'night'.
'night'+'ie'='NIGHTIE'
(Other definitions for nightie that I've seen before include "put on at end of day?" , "Light sleeping gown" , "Clothing worn in bed" , "Loose dress worn in bed" , "Sleeping garment" .)