You could try this:
SELECT z.Sno, z.LeadId, z.[Tenure Source], z.Premium
FROM
(
SELECT x.Sno, x.LeadId, x.[Tenure Source], x.Premium
FROM
(
SELECT ROW_Number() Over (ORDER By Sno, LeadId) as Rowno,
Sno, LeadId, [Tenure Source], dc.premiumAmount as Premium
FROM tbl_LeadCampaignData_Prospect lcdp
LEFT JOIN bl_GI_DC_NSTP dc on dc.LeadID=lcdp.LPID
) as x
WHERE x.Rowno = 1
UNION
SELECT y.Sno, y.LeadId, y.[Tenure Source], y.Premium
FROM
(
SELECT ROW_Number() Over (ORDER By Sno, LeadId) as Rowno,
Sno, LeadId, [Tenure Source], NULL as Premium
FROM tbl_LeadCampaignData_Prospect lcdp
) as y
WHERE y.Rowno > 1
) as z
ORDER BY z.Sno, z.LeadId