Fancy horse that's catching leader in race (6)
I believe the answer is:
frilly
'fancy' is the definition.
(I've seen this before)
'horse that's catching leader in race' is the wordplay.
'horse' becomes 'filly' (I have seen 'Female horse ' mean 'filly' so perhaps 'horse' could also mean 'filly').
'that's catching' is an insertion indicator.
'leader in race' becomes 'r' (I've seen this in other clues).
'filly' going around 'r' is 'FRILLY'.
(Other definitions for frilly that I've seen before include "Fancy" , "ruffled" , "Having flounces, ruffles, etc" .)