Go "Boo-hoo!" (4)
'go boo-hoo' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sob that I've seen before include "Weep noisily" , "Weep heavily, wail" , "Blubber" , "Wep convulsively" , "Weep audibly" .)
(Other definitions for cry that I've seen before include "Don't . . . over spilt milk" , "Blubber" , "Sound of pain or grief" , "Call" , "produce tears" .)
(Other definitions for weep that I've seen before include "pipe" , "Cry, shed tears" , "Turn on the waterworks" , "Seep slowly out" , "lament" .)