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 weep that I've seen before include "leak" , "Express grief or sorrow" , "Greet" , "Sob - ooze" , "lament" .)