Stay calm and don't undress! (4,4,5,2)
I believe the answer is:
keep your shirt on
'stay calm' is the definition.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for keep your shirt on that I've seen before include "Unlike a ruined gambler" , "Avoid exposing body to sun's rays" , "remain cool" , "don't panic!" , "Not topless please" .)