Ruffled young horse hugs rail at the start (6)
I believe the answer is:
frilly
'ruffled' is the definition.
(I know that ruffled can be written as frilly)
'young horse hugs rail at the start' is the wordplay.
'young horse' becomes 'filly' (I've seen this before).
'hugs' indicates putting letters inside.
'at the start' says to take the initial letters.
The first letter of 'rail' is 'r'.
'filly' going around 'r' is 'FRILLY'.
(Other definitions for frilly that I've seen before include "Fancy" , "ruffled" , "Having flounces, ruffles, etc" .)