Toss head of radicchio in casserole (5)
I believe the answer is:
strew
'toss' is the definition.
(thesaurus)
'head of radicchio in casserole' is the wordplay.
'head of' indicates taking the first letters.
'in' indicates putting letters inside.
'casserole' becomes 'stew' (I've seen this before).
The first letter of 'radicchio' is 'r'.
'r' placed into 'stew' is 'STREW'.
(Other definitions for strew that I've seen before include "Scatter loosely" , "Sprinkle" , "Scatter, spread around" , "Scatter untidily" .)