Part of a mushroom (4)
I believe the answer is:
stem
'part of a mushroom' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for stem that I've seen before include "Thwart; support" , "supporter of flag, for example" , "feature of wine glass" , "Supporting structure" , "'Stalk, shank (4)'" .)