One, commonly halved (4)
I believe the answer is:
only
'one' is the definition.
(synonyms)
'commonly halved' is the wordplay.
'halved' means to take half the letters.
'commonly' cut in half is 'ONLY'.
(Other definitions for only that I've seen before include "Exclusive - individual" , "And nothing more; sole" , "Existing alone with no other, perhaps a child" , "Alone; solitary" , "Single, solitary" .)