Scatter a little rosemary in casserole (5)
I believe the answer is:
strew
'scatter' is the definition.
(thesaurus)
'a little rosemary in casserole' is the wordplay.
'a little' says to take the initial letters.
'in' means one lot of letters goes inside another.
'casserole' becomes 'stew' (I've seen this before).
The first letter of 'rosemary' is 'r'.
'r' put within 'stew' is 'STREW'.
(Other definitions for strew that I've seen before include "Scatter loosely" , "Scatter untidily" , "Scatter, spread around" , "Sprinkle" .)