Achieved this by using below function
create function like below
###############################################
function ClearAssingeeField() {
document.getElementById(assignee_id).value="";
###############################################
call this function
<body onload="load_from_scratchpad('0');initApcForNewTicket() ;ClearAssigneeField();
<PDM_INCLUDE FILE=std_body.htmpl>
Add var assignee_id=_dtl.currID; before"NEW Assignee"Field lines like below
<PDM_IF "$args.type" == "R" || "$args.type" == "">
<PDM_MACRO name=dtlLookup hdr="NEW Assignee" attr=assignee make_required=yes>
<PDM_ELSE>
<PDM_MARCO name=dtlLookup hdr="NEW Assignee" attr=assignee>
var assignee_id=_dtl.currID;
/PDM_IF>
<PDM_ELIF "$args.type"=="I" && "$env.NX_REQUIRE_INCIDENT_ASSIGNEE"=="Yes">
<PDM_MACRO name=dtlLookup hdr="NEW Assignee" attr=assignee make_required=yes>
var assignee_id=_dtl.currID;
<PDM_ELIF "$args.type"=="P" && "$env.NX_REQUIRE_PROBLEM_ASSIGNEE"=="Yes">
<PDM_MACRO name=dtlLookup hdr="NEW Assignee" attr=assignee make_required=yes>
var assignee_id=_dtl.currID;
<PDM_ELSE>
<PDM_MARCO name=dtlLookup hdr="NEW Assignee" attr=assignee>
</PDM_IF>
var assignee_id=_dtl.currID;