But commonly halved (4)
I believe the answer is:
only
'but' is the definition.
(I've seen this in another clue)
'commonly halved' is the wordplay.
'halved' indicates taking half.
'commonly' cut in half is 'ONLY'.
(Other definitions for only that I've seen before include "Unique; merely" , "See 6" , ". . . . the Lonely (sung by Roy Orbison)" , "Except that; single" , "And nothing more; sole" .)