Garment: tailored thing, that is (7)
I believe the answer is:
nightie
'garment' is the definition.
(I've seen this before)
'tailored thing that is' is the wordplay.
'tailored' indicates anagramming the letters (the letters are customised ).
'that is' becomes 'ie' (synonyms).
'thing' anagrammed gives 'night'.
'night'+'ie'='NIGHTIE'
(Other definitions for nightie that I've seen before include "one put on after dark" , "Skimpy attire" , "Bedtime wear" , "Bedtime attire" , "What retiring lady assumes" .)