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.

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.