Extra large mushroom (5)
I believe the answer is:
morel
'mushroom' is the definition.
(I know that morel is a type of mushroom)
'extra large' is the wordplay.
'extra' becomes 'more' (both can mean additional).
'large' becomes 'L' (eg in clothes sizes).
'more'+'l'='MOREL'
(Other definitions for morel that I've seen before include "Edible mushroom" , "One has a cap" , "Edible fungus" , "Type of mushroom" .)