Here is the context said by a boy who was training in boxing.
左ジャブで敵の体勢をくずし 突破口を見いだしたらば すかさず右ストレートを打つべし これ拳闘の攻撃における基本なり...とくら
I have found some sites explaining that it is changed from と来る but I still don't get it.
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「～～とくら」 is a colloquial and masculine Tokyo way of pronouncing 「～～と + くる + わ」. The contraction just so quintessentially sounds Tokyo.
(Unlike what so many J-learners seem to firmly believe, this 「わ」 is not a feminine sentence-ender.)
I am going to call this 「と」 quotative just because there is no other explanation that seems feasible in my brain.
「～～とくら」, in OP's context, is indeed difficult to explain as it is far more nuanced than the words in the phrase would suggest. If I had to attempt a free translation, I would probably go with expressions such as:
"~~ is the attitude I am going to take"
"~~ is what I want to declare"
"~~ is what I believe"
I said the 「と」 was quotative because the speaker is quoting his own opinion, belief, etc. here.
It's slurred …と来るわ (or possibly …と来れば but in this case it's the former, if I remember correctly).
「とくら」is a colloquial deformation of 「… とくるは」meaning ① “speaking of,” ② “It comes out as …, Its outcome is …” and ③ “in addition.”
In 江戸っ子弁 – Edo (Tokyo)-ite parlance, it has been used to be pronounced as […とくらア].
It was used in such a way;
①あいつとくら、いつも嘘（うそ）ばかり言ってやがる – (Speaking of him) he’s telling a lie always.
②あいつは性根（しょうね）が悪い上に、骨（ほね）の髄（ずい）までド吝（けち）とくらあ、仕様（しよう）のねえ野郎（やろう）だ - He is not only ill-natured, but stingy to his bones. He’s hopeless.
③上の娘（むすめ）の結婚式（けっこんしき）の上に、息子（むすこ）の大学進学（だいがくしんがく）とくら、今年（ことし）は物入（ものい）りだ – My elder daughter marries. (In addition) my son enters the university. I’m really in trouble with a lot of expenditures this year.