Intervene (4,2)
'intervene' is the definition.
(I know that intervene can be written as step in)
This is the entire clue.
(Other definitions for step in that I've seen before include "Substitute" , "Intervene dramatically, or at a senior level" , "Interrupt" , "Type of garment" , "Shoe with no fastenings" .)
'intervene' is the definition.
(to mediate is to intervene between two parties)
This is all the clue.
(Other definitions for mediate that I've seen before include "Act as negotiator between opposing side" , "Act as intermediary" , "ie Tamed (anag.)" , "Conciliate" , "moderate" .)