Nail one inside for decoration (5)
I believe the answer is:
braid
'decoration' is the definition.
(braid is a kind of decoration)
'nail one inside' is the wordplay.
'nail' becomes 'brad' (brad is a kind of nail).
'one' becomes 'i' (Roman numeral).
'inside' indicates putting letters inside.
'brad' going around 'i' is 'BRAID'.
'for' is the link.
(Other definitions for braid that I've seen before include "Decorative ribbon" , "As before, reproach" , "Ornamental tape or plait" , "Hairdo formed by twisting the hair" , "Plaited length, of hair say." .)