Fool's step up (3)
I believe the answer is:
sap
'fool's' is the definition.
(synonyms)
'step up' is the wordplay.
'step' becomes 'pas' (I've seen this before).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'pas' back-to-front is 'SAP'.
(Other definitions for sap that I've seen before include "Moisture in tree or informal dunce" , "Drain - vital fluid" , "Plant juice; undermine" , "Insidiously undermine" , "Imbecile" .)