Fool horse with backwards look (4)

I believe the answer is:

clot

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'fool' is the definition.
(I've seen this before)

'horse with backwards look' is the wordplay.
I cannot quite see how this works, but
'look' could be 'lo' ('lo' is an archaic exclamation meaning 'look!') and 'lo' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for clot that I've seen before include "medical problem" , "(Of blood) coagulate" , "3 [DUMMY]" , "Semi-solid mass -- dunce" , "Curdle - idiot" .)

I've seen this clue in The Guardian.

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