Squeeze hard cracking nuts on horse (8)
I believe the answer is:
shoehorn
'squeeze' is the definition.
(both can mean to force something in)
'hard cracking nuts on horse' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'cracking' means one lot of letters goes inside another (inserted letters go into a gap or crack).
'nuts' is an anagram indicator (nuts can mean crazy).
'on'+'horse'='onhorse'
'onhorse' anagrammed gives 'shoeorn'.
'h' put inside 'shoeorn' is 'SHOEHORN'.
(Other definitions for shoehorn that I've seen before include "It facilitates getting foot into footwear" , "A fitting device" , "to make output of last worker yield more" , "What wife too big for her boots may need" , "Aid to putting on footwear" .)