Mushroom top (3)
I believe the answer is:
cap
'mushroom top' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cap that I've seen before include "'If the . . . fits, wear it' (3" , "Surpass with headwear" , "Limit (prices)" , "Soft headgear" , "One on head; excel" .)