Stop in the shade, terribly hot (5)
I believe the answer is:
deter
'stop' is the definition.
(to deter behaviour is to stop it)
'in the shade terribly hot' is the wordplay.
'in' indicates the answer is hidden within the clue.
'DETER' is hidden in the letters of 'shade terribly hot'.
(Other definitions for deter that I've seen before include "Warn off" , "Discourage (someone from an action)" , "Hinder, discourage" , "Dishearten" , "Daunt" .)