Horse died — Sue admits boundless regret (6,4)

'horse' is the definition.
(type of horse)

'died sue admits boundless regret' is the wordplay.
'died' becomes 'd' (abbreviation next to the year of someone's death).
'sue' becomes 'apply' (I've seen this before).
'admits' means one lot of letters goes inside another.
'boundless' indicates the central letters (the limits or bounds of the word are removed).
The middle letters of 'regret' are 'egre'.
'apply' placed around 'egre' is 'applegrey'.
'd'+'applegrey'='DAPPLE-GREY'