Wiktionary
crack a smile
vb. 1 (context idiomatic English) To smile, especially while attempting not to. 2 (context idiomatic English) To begin to smile.
Usage examples of "crack a smile".
Ser Alliser Thorne was the only man at table who did not so much as crack a smile.
When he straightened up, he saw a trace of humor in the old king's eyes--but the royal face didn't crack a smile, much less start talking.
The men had heard him say this before, and knew better than to crack a smile.