Hide anger with impression (4)
I believe the answer is:
idea
'impression' is the definition.
(I've seen this before)
'hide anger with' is the wordplay.
'with' indicates the answer is hidden within the clue.
'IDEA' can be found hidden inside 'hide anger'.
(Other definitions for idea that I've seen before include "Mental impression" , "Brainwave" , "Notion, concept" , "Sudden inspiration?" , "Plato's Form is" .)