Speak softly, literally. Calm down! (4,2)
I believe the answer is:
cool it
'calm down' is the definition.
(synonyms)
'speak softly literally' is the wordplay.
'speak softly' becomes 'coo' (cooing is a kind of speaking softly).
'literally' becomes 'lit' (I've seen this in other clues).
'coo'+'lit'='COOL IT'
(Other definitions for cool it that I've seen before include "Modern expression for calm down, or reduce heat" , "Here's what to do before opening champagne" , "Don't get so excited'" , "In slang, calm down" , "Modern injunction to calm down" .)