Fool that is taking in crazy climb across end of ridge (8)
I believe the answer is:
imbecile
'fool' is the definition.
(thesaurus)
'that is taking in crazy climb across end of ridge' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'taking in' is an insertion indicator.
'crazy' indicates an anagram.
'across' means one lot of letters goes inside another.
'end of' says to take the final letters.
The final letter of 'ridge' is 'e'.
'climb' with letters rearranged gives '[climb]'.
'ie' placed around '[climb]' is 'i[climb]e'.
'imbcile' enclosing 'e' is 'IMBECILE'.
(Other definitions for imbecile that I've seen before include "Abusive word for foolish person" , "Charlie" , "Dolt" , "Sap" , "Moron or cretin" .)