Initials slowly put upon rear boot spike (4)

I believe the answer is:

spur

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'boot spike' is the definition.
'spur' can be an answer for 'spike' (thesaurus). I'm not certain of the 'boot' bit.

'initials slowly put upon rear' is the wordplay.
'initials' indicates taking the first letters.
The initial letters of 'slowly put upon rear' is 'SPUR'.

Can you help me to learn more?

(Other definitions for spur that I've seen before include "Spike on a horse-rider's boot" , "branch road" , "Goad - branch line" , "Sharp pointed projection" , "Goad; stimulus" .)

I've seen this clue in The Mirror.

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