Clean up Toby's first belch (5)
I believe the answer is:
erupt
'belch' is the definition.
(I know that belch can be written as erupt)
'clean up toby's first' is the wordplay.
'clean' becomes 'pure' (similar in meaning).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'toby's first' becomes 't' (1st letter of 'toby').
'pure' backwards is 'erup'.
'erup'+'t'='ERUPT'
(Other definitions for erupt that I've seen before include "Spurt lava" , "Break out like Etna" , "Discharge" , "Break out dramatically" , "Emit lava" .)