Mad, reversing over mountain pass (4)
I believe the answer is:
loco
'mad' is the definition.
(thesaurus)
'reversing over mountain pass' is the wordplay.
'reversing' shows that the letters should be reversed in order.
'over' becomes 'o' (cricket abbreviation).
'mountain pass' becomes 'col' (col is a kind of mountain pass**).
'o'+'col'='ocol'
'ocol' written backwards gives 'LOCO'.
(Other definitions for loco that I've seen before include "plant increasing milk yield" , "Round the bend" , "Crazy in America - cool, man" , "Crazy; engine" , "Railway engine - mad!" .)