Something cold noticed inside (3)
I believe the answer is:
ice
'something cold' is the definition.
'noticed inside' is the wordplay.
'inside' indicates the answer is hidden within the clue.
'ICE' is hidden in the letters of 'noticed'.
(Other definitions for ice that I've seen before include "Rocks on which drink is served?" , "Diamonds, colloquially" , "Decorate a cake" , "Diamonds (sl)" , "Diamonds (informal)" .)