Climbing in the foothills, excessively (3)
I believe the answer is:
too
'excessively' is the definition.
(to too great a degree)
'climbing in the foothills' is the wordplay.
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'in' indicates the answer is hidden within the clue.
'oot' is hidden within 'foothills'.
'oot' written backwards gives 'TOO'.
(Other definitions for too that I've seen before include "Likewise" , "Unduly" , "Excessively" , "As well; in addition" , "To a greater than desirable extent" .)