Mug takes a step back (3)
I believe the answer is:
sap
'mug' is the definition.
(thesaurus)
'a step back' is the wordplay.
'a step' becomes 'pas' (I've seen this in another clue).
'back' shows that the letters should be reversed in order.
'pas' reversed gives 'SAP'.
'takes' acts as a link.
(Other definitions for sap that I've seen before include "Exhaust" , "Liquid that circulates in plants" , "tunnel" , "Tree's blood" , "Watery substance in a plant" .)