It's cold heart of reticence (3)
I believe the answer is:
ice
'it's cold' is the definition.
(I've seen this before)
'heart of reticence' is the wordplay.
'heart of' indicates the answer is hidden within the clue.
'ICE' is hidden in the letters of 'reticence'.
(Other definitions for ice that I've seen before include "Diamond" , "Rocks on which drink is served?" , "Decorate a cake" , "Diamonds, colloquially" , "Skating surface" .)