Royal seen among tree ferns ordered snack (11)
I believe the answer is:
refreshment
'snack' is the definition.
(refreshment is a kind of snack)
'royal seen among tree ferns ordered' is the wordplay.
'royal' becomes 'hm' (His or Her Majesty).
'seen among' means one lot of letters goes inside another.
'ordered' is an anagram indicator.
'tree'+'ferns'='treeferns'
'treeferns' anagrammed gives 'refresent'.
'hm' placed within 'refresent' is 'REFRESHMENT'.
(Other definitions for refreshment that I've seen before include "Renewal, reinvigoration" , "Light snack or drink" , "Food or drink" , "something to take down?" , "stimulation" .)