Criticise severely when stopping nonsense (5)
I believe the answer is:
roast
'criticise severely' is the definition.
(I've seen this before)
'when stopping nonsense' is the wordplay.
'when' becomes 'as'.
'stopping' indicates putting letters inside (stopping can mean blocking a gap).
'nonsense' becomes 'rot' (I've seen this before).
'as' inserted within 'rot' is 'ROAST'.
(Other definitions for roast that I've seen before include "Swelter" , "Traditional Sunday joint?" , "Heavily criticise" , "Cooked meat; criticise" , "cooked a certain way" .)