Disregard “X” placed under warrant (4,2)
I believe the answer is:
pass by
'disregard' is the definition.
(thesaurus)
'x placed under warrant' is the wordplay.
'x' becomes 'by' (both can mean multiplied by).
'placed under' means one lot of letters go next to another.
'warrant' becomes 'pass'.
'by' put after 'pass' is 'PASS BY'.
(Other definitions for pass by that I've seen before include "Go past - disregard" , "''Cast a cold eye / On life, on death / Horseman ... ... (WB Yeats)" , "Leave on one side" , "Move along in the vicinity" , "neglect" .)