Step back from a trench (3)
I believe the answer is:
sap
'a trench' is the definition.
(thesaurus)
'step back' is the wordplay.
'step' becomes 'pas' (I've seen this before).
'back' says the letters should be written in reverse.
'pas' back-to-front is 'SAP'.
'from' is the link.
(Other definitions for sap that I've seen before include "Plant juice; undermine" , "Exhaust" , "tunnel" , "The juice in trees" , "Undermine; fool" .)