Crossword Genius

Head sorry about mammal (4,5)

Ross

I believe the answer is:

dray horse

I'm a little stuck... Click here to teach me more about this clue!

'mammal' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Perhaps there's an association between them I don't understand?

'head sorry about' is the wordplay.
'about' is an anagram indicator.
'head'+'sorry'='headsorry'
'headsorry' with letters rearranged gives 'DRAY-HORSE'.

Can you help me to learn more?

(Another definition for dray horse that I've seen is " load-bearer".)

I've seen this clue in The Mirror.
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