Calm down a beat, somehow (5)
I believe the answer is:
abate
'calm down' is the definition.
(I've seen this in another clue)
'a beat somehow' is the wordplay.
'somehow' indicates an anagram.
'beat' is an anagram of 'bate'.
'a'+'bate'='ABATE'
(Other definitions for abate that I've seen before include "Wane" , "Become less strong like storm" , "Demolish" , "Let up or die away" , "drop off" .)