You shouldn’t have stood up periodically hiding something up your sleeve? (5,3)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

definition: “You shouldn’t have” as an expression of gratitude periodic letters from [s]-T-[o]-O-[d] U-[p]; around/”hiding” HANKY (a handkerchief, that might be kept up one’s sleeve)