Grammatically, it's a subsidiary verb. Semantically, 明鏡国語辞典 defines this as follows:
のける
《動詞連用形+接続助詞「て」について》やりにくいことを平気で、またみごとにしてしまう意を表す。「外聞の悪いことを平然と言って―」「たった一晩で修復工事をやって―」
デジタル大辞泉's definition is almost the same:
動詞の連用形に接続助詞「て」を添えた形に付いて、やりにくいことをみごとに、また、思いきってやってしまう意を表す。「少ない日程で作業をやって―・ける」「言いたいことを相手構わず言って―・ける」
Here, やりにくいこと ("difficult thing to do") can be either something that is technically difficult or something that is socially/ethically inappropriate or questionable.
- When it is used with a technically difficult thing, it means "to really do ~", "to succeed in doing ~", etc. It sometimes (but not always) has the connotation of "easily" and used with words like 平然と, こともなげに, 当然のように, etc.
- When it is used with a ethically/socially questionable thing, it means "dare do ~", "to really do ~", "to have the courage to do ~", "to be bold enough to ~", "not to hesitate to do ~", etc. Can be used for both praising and scorning someone.
The subject is almost always a human. その車は1時間で東京まで走ってのけた sounds a little unnatural to me.
You can see many examples here: http://yourei.jp/%E3%81%A6%E3%81%AE%E3%81%91%E3%82%8B