On top of it all, a mushroom (2,4)
I believe the answer is:
as well
'on top' is the definition.
(I've seen this before)
'a mushroom' is the wordplay.
'mushroom' becomes 'swell' (both can mean to expand or grow in size).
'a'+'swell'='AS WELL'
'of it all' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for as well that I've seen before include "Also, additionally" , "To boot" , "Also, in addition" , "A swell sort of addition" , "Appropriate" .)