Decoration of help to British Rail? (5)
I believe the answer is:
braid
'decoration' is the definition.
(braid is a kind of decoration)
'help to british rail?' is the wordplay.
'help' becomes 'aid' (synonyms).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'british rail?' becomes 'br'.
'aid' put after 'br' is 'BRAID'.
'of' is the link.
(Other definitions for braid that I've seen before include "Gilded ribbon" , "Decorative ribbon" , "woven fabric" , "Hairdo formed by twisting the hair" , "Intertwine (hair)" .)