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.
Japanese Language Stack Exchange is a question and answer site for students, teachers, and linguists wanting to discuss the finer points of the Japanese language. It only takes a minute to sign up.
Sign up to join this community「~~とくら」 is a colloquial and masculine Tokyo way of pronouncing 「~~と + くる + わ」. This contraction just so quintessentially sounds Tokyo, working-class and "tough-guy-like".
(Unlike what 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 used 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.
「とくら」is a colloquial deformation of 「… とくるは」meaning ① “speaking of,” ② “It comes out as …, It's outcome is …” and ③ “in addition.”
In 江戸っ子弁 – Edo (Tokyo)-ite parlance, it used to be pronounced […とくらア].
It was used in the following ways:
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 always telling lies.
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 is getting married. (In addition) my son is going to university. I’m really in trouble with a lot of expenditures this year.
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).