È necessario il raggruppamento per le colonne non aggregate
SELECT tab_sector.sector_id,tab_sector.sector_title,tab_sector.sector_desc
,tab_sector.sector_image,group_concat(tab_sector_subdivisions.subdiv_id )
LEFT JOIN tab_sector_subdivisions
ON tab_sector_subdivisions.sector_id = tab_sector.sector_id
AND tab_sector.active = 'Y'
GROUP BY tab_sector.sector_id,tab_sector.sector_title,
tab_sector.sector_desc, tab_sector.sector_image