Green’s Dictionary of Slang

clutch (up) v.

(US) to freeze up under pressure.

Science Digest Aug. 34: ‘Clutch’ [...] to be nervous or upset before, during or after any occasion of importance [HDAS].
[US]Current Sl. I:3 2/1: To fail completely under duress.
[US]Current Sl. III:1 5: Clutch, v. To panic or become tense.
E. Leonard Big Bounce 123: [I]t wasn’t a game; it was real and maybe she could do it without clutching up, but how did she know until she had done it.
[US]D. Hall Dock Ellis 34: [T]he superb athlete who never made it, because he clutched; who would excel in practice or in warm-up, but in the heat of battle drop the ball.

In derivatives

clutched (up) (adj.)

(US) frightened, nervous, tense, awed, thus v. clutch up, to render tense.

Cope & Dyer Petty Officer’s Guide 427: Clutched. (slang). All tensed up [HDAS].
[US](con. 1950) E. Frankel Band of Brothers 3: ‘Thought I saw something . . . like a shadow . . . movin’ . . . Goober and Firesteen [...] stared at the village. ‘You’re clutched up,’ Goober said. ‘I don’t see a thing.’.
[US]G. Swarthout Where the Boys Are 5: If you come from the Midwest and have never seen it you are really clutched, that is, seized by emotion. [Ibid.] 138: I was double-clutched. I had no clue whether to laugh ... or cry.
[US]D. Wakefield Going All the Way 244: He wasn’t clutched up at all [...] He hadn’t even got tongue-tied.
E. Leonard Cat Chaser 160: If that’s what’s got you clutched up, don’t worry about it.