Little nail holding one decorative band (5)
I believe the answer is:
braid
'decorative band' is the definition.
(band around an item of clothing)
'little nail holding one' is the wordplay.
'little nail' becomes 'brad' (brad is a type of nail).
'holding' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'brad' placed around 'i' is 'BRAID'.
(Other definitions for braid that I've seen before include "Gilded ribbon" , "woven fabric" , "Decorative band for trimming" , "jerk" , "Ornamental strip" .)