Bit of fungus enveloping root of sycamore (6)
I believe the answer is:
morsel
'bit' is the definition.
(synonyms)
'fungus enveloping root of sycamore' is the wordplay.
'fungus' becomes 'morel' (morel is a kind of fungus).
'enveloping' is an insertion indicator.
'root of sycamore' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'morel' enclosing 's' is 'MORSEL'.
'of' is the link.
Can you help me to learn more?
(Other definitions for morsel that I've seen before include "(Tasty) bit" , "A scrap or shred of food" , "A small amount of food, just a piece" , "Dainty" , "Tiny piece" .)