Tiny bit of doughnut eaten by Buttrose (4)
I believe the answer is:
iota
'tiny bit' is the definition.
(an iota is a tiny piece)
'doughnut eaten by buttrose' is the wordplay.
I cannot quite understand how this works, but
'doughnut' could be 'o' (letter resembling a doughnut) and 'o' is present in the answer.
The remaining letters 'ita' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for iota that I've seen before include "Tiniest scrap" , "Contrivance" , "little part" , "European letter" , "Grain" .)