Garment that is dark to begin with (7)
I believe the answer is:
nightie
'garment' is the definition.
(I've seen this before)
'that is dark to begin with' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'dark' becomes 'night' (it's dark at nighttime).
'to begin with' is a charade indicator (letters next to each other).
'ie' after 'night' is 'NIGHTIE'.
(Other definitions for nightie that I've seen before include "Bedtime habit" , "Clothing worn in retirement?" , "Sleeping garment" , "Coverage for the rest" , "Skimpy attire" .)