King perhaps eating hard vegetable (5)
I believe the answer is:
chard
'vegetable' is the definition.
(chard is a kind of vegetable)
'king perhaps eating hard' is the wordplay.
'king perhaps' becomes 'card' (king is a kind of card).
'eating' means one lot of letters goes inside another.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'card' enclosing 'h' is 'CHARD'.
(Other definitions for chard that I've seen before include "Beet with edible leaves and stalks" , "Kind of (Swiss?) beet with edible leaves" , "Leaf stalks from a kind of beet" , "salad" , "vegetable crop" .)