Give way under pressure (4,2)
(Other definitions for cave in that I've seen before include "Lose willpower" , "Subside, collapse" , "Collapse of, eg, tunnel" , "Yield to outside pressure" , "(Roof) collapse; abandon resistance" .)
(Other definitions for buckle that I've seen before include "Crumple under pressure, or belt fastener" , "Fasten up - give way" , "Crumple under pressure, like shoe clasp" , "Catch" , "Fastener on belt or shoe" .)