Fastening horse into strap (9)
I believe the answer is:
shackling
'fastening' is the definition.
'horse into strap' is the wordplay.
'horse' becomes 'hack' (hack is a kind of horse**).
'into' is an insertion indicator.
'strap' becomes 'sling'.
'hack' put into 'sling' is 'SHACKLING'.
(Another definition for shackling that I've seen is " Restraining".)
I've seen this clue in The Guardian.
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