Extra large fungus (5)
I believe the answer is:
morel
'fungus' is the definition.
(morel is a kind of fungus)
'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 "One has a cap" , "Type of mushroom" , "Edible mushroom" , "Edible fungus" .)