Difficulty slotting donkey's head into horse's straps (8)

I believe the answer is:

hardness

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

'difficulty' is the definition.
(I know that hardness is a type of difficulty)

'donkey's head into horse's straps' is the wordplay.
'head' says to take the initial letters.
'into' indicates putting letters inside.
'horse' becomes 'H' (abbreviation as in horse power).
'straps' becomes 'arness' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The first letter of 'donkeys' is 'd'.
'h'+'arness'='harness'
'd' put within 'harness' is 'HARDNESS'.

'slotting' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for hardness that I've seen before include "Unyielding quality" , "Rigidity, firmness" , "Characteristic of mineral-rich water" .)

I've seen this clue in the Sydney Morning Herald.

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