Astonish with crazy climbing (4)
I believe the answer is:
stun
'astonish' is the definition.
(thesaurus)
'crazy climbing' is the wordplay.
'crazy' becomes 'nuts' (I've seen this before).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'nuts' backwards is 'STUN'.
'with' acts as a link.
(Other definitions for stun that I've seen before include "'Shock, daze (4)'" , "Stagger, amaze" , "Render semi-conscious with blow" , "Shock into silence" , "Take aback" .)