Time with Heather's racehorses (9)
I believe the answer is:
yearlings
'racehorses' is the definition.
(yearling is a kind of racehorse)
'time with heather's' is the wordplay.
'time' becomes 'year' (I've seen this before).
'with' means one lot of letters go next to another.
'heather's' becomes 'lings' ('ling' can be a synonym of 'heather').
'year'+'lings'='YEARLINGS'
(Other definitions for yearlings that I've seen before include "Young horses" , "young runners" , "Racehorses in their second year" .)