Sentence:  "Why on earth do you <WSD>go in for</WSD> track?" I asked him. And then he explained that he thought that, if you wanted to be an all-around man, you ought to cultivate some form of athletics— he's actually taken up pole-vaulting: isn't that a ghastly thought? 
Meaning of the word marked with <WSD> tags:  To engage oneself or take part in something.