Elves seen cooking snack (9)
I believe the answer is:
elevenses
'snack' is the definition.
(I've seen this before)
'elves seen cooking' is the wordplay.
'cooking' indicates anagramming the letters.
'elves'+'seen'='elvesseen'
'elvesseen' anagrammed gives 'ELEVENSES'.
(Other definitions for elevenses that I've seen before include "tenses found immediately before this?" , "Early drink" , "Morning snack" , "Morning refreshment break" , "Tea and biscuits, perhaps" .)