Fifty guided up valley (4)
I believe the answer is:
dell
'valley' is the definition.
(I know that dell is a type of valley)
'fifty guided up' is the wordplay.
'fifty' becomes 'l' (Roman numeral).
'guided' becomes 'led' (similar in meaning).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'l'+'led'='lled'
'lled' written backwards gives 'DELL'.
(Other definitions for dell that I've seen before include "Small valley, rhymes with tumbled down" , "Small secluded valley" , "Manufacturer that relies heavily on Silicon" , "Little wooded hollow" , "Small wooded hollow or valley" .)