I think the reason "defensive clutch" isn't spoken of is because people look at defense as a "less precise" art. People look at shooting as being *VERY* precise - right form, right arch, good release, quick release, proper distance, proper angle...even at proper time. People don't look at getting a steal the same way. It doesn't have to be *just* *so*. Anything that works, works. Anything sufficiently close probably works well enough.
I also don't think you hear about "clutch passers," either - people who instinctively get the ball to the right person at the right time. Why? Because, as with defense, they see that as an imprecise art.
The idea that the art is imprecise means that it is less affected by outside factors (fatigue, focus, physical effects of andrenaline or epinephrine, etc.). Since it's less affected by outside factors, it's not as difficult. Less difficult means less valued. At least, that's my theory.
Then again - how many "clutch" defensive plays can you name? There's "Havlicek stole the ball!" There's the Bird steal and pass to Dennis Johnson, but even that adds in that offensive component - and it's the only "clutch pass" that comes to mind.
Finally, another reason defense isn't looked at as significantly is because defense doesn't CHANGE the outcome of the game; it "seals" it, because defense in basketball doesn't score points. And if a guy picks of a steal and goes in for a lay-up - well...he's SUPPOSED to hit his lay-ups, so that couldn't possibly be clutch, could it? No defender changes his situation from one of losing to one of winning without a following offensive play of some type. (Football is the single obvious exception, and that's because defenses can score points in football.)