Extremely palatable, a vegetable (3)
I believe the answer is:
pea
'vegetable' is the definition.
(pea is a type of vegetable)
'extremely palatable a' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'palatable' with its middle removed is 'pe'.
'pe'+'a'='PEA'
(Other definitions for pea that I've seen before include "Climber" , "Legume" , "Leguminous plant" , "registered by princess in bed" , "Climbing plant; vegetable" .)