Put it under your pillow (7)
I believe the answer is:
bolster
'pillow' is the definition.
(bolster is a kind of pillow)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for bolster that I've seen before include "Chisel - long cushion" , "Support - strengthen" , "Reinforce; a cushion" , "Support; long pillow" , "Prop up with long pillow" .)