modelka,
tags у вас массив !!!
var json = {
"data": {
"success": true,
"errors": [],
"response": {
"158719": {
"id": 158719,
"article": 158719,
"constructor_id": null,
"name": "\u0417\u0430\u0441\u0442\u0440\u0435\u043b\u0438\u0441\u044c!",
"description": "",
"seo_title": "",
"seo_description": "",
"seo_keywords": "",
"tags": [{
"id": "645",
"tkey": "devushkam"
}]
}
}
}
} ;
alert(json.data.response[158719].tags[0].id)