Small piece of mushroom without top of stalk (6)
I believe the answer is:
morsel
'small piece' is the definition.
(I've seen this before)
'mushroom without top of stalk' is the wordplay.
'mushroom' becomes 'morel' (morel is a kind of mushroom).
'without' means one lot of letters goes inside another ('without' can be similar in meaning to 'outside').
'top of' says to take the initial letters.
The initial letter of 'stalk' is 's'.
'morel' going around 's' is 'MORSEL'.
'of' acts as a link.
(Other definitions for morsel that I've seen before include "A small amount of food, just a piece" , "Titbit" , "Part" , "A scrap or shred of food" , "Small bit of food" .)