Loose garment, almost secure (7)
I believe the answer is:
nightie
'loose garment' is the definition.
(I've seen this before)
'almost secure' is the wordplay.
'almost' becomes 'nigh' (nigh can mean almost or nearly).
'secure' becomes 'tie' (tying is a kind of securing).
'nigh'+'tie'='NIGHTIE'
(Other definitions for nightie that I've seen before include "Lingerie worn in bed by women" , "Bedtime habit" , "Female article of clothing" , "Bed attire (colloq.)" , "Familiar bed-wear for woman" .)