Extra mushroom left out (4)
I believe the answer is:
more
'extra' is the definition.
(I've seen this before)
'mushroom left out' is the wordplay.
'mushroom' becomes 'morel' (morel is a kind of mushroom).
'left' becomes 'l' (common abbreviation).
'out' indicates named letters should be taken away.
'morel' with 'l' taken away is 'MORE'.
(Other definitions for more that I've seen before include "To a greater extent" , "victim of Henry" , "See 9" , "Greater quantity or number" , "Additional or larger amount" .)