Diminishes a beast, somehow (6)
I believe the answer is:
abates
'diminishes' is the definition.
(abating is a kind of diminishing)
'a beast somehow' is the wordplay.
'somehow' indicates an anagram.
'beast' anagrammed gives 'bates'.
'a'+'bates'='ABATES'
(Other definitions for abates that I've seen before include "has a fall" , "Slacks off" , "moderates" , "Becomes less severe or strong" , "Diminishes" .)