Elves seen preparing snack (9)
I believe the answer is:
elevenses
'snack' is the definition.
(I've seen this before)
'elves seen preparing' is the wordplay.
'preparing' is an anagram indicator (the same letters prepared differently).
'elves'+'seen'='elvesseen'
'elvesseen' anagrammed gives 'ELEVENSES'.
(Other definitions for elevenses that I've seen before include "Coffee time?" , "Early drink" , "Tea and biscuits, perhaps" , "Morning tea or coffee break" , "Soccer teams" .)