Part of Polynesia invaded by small grub (6)
I believe the answer is:
samosa
'grub' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'part of polynesia invaded by small' is the wordplay.
'part of polynesia' becomes 'Samoa' ('Samoa' is part of 'polynesia').
'invaded by' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'samoa' going around 's' is 'SAMOSA'.
Can you help me to learn more?
(Other definitions for samosa that I've seen before include "Grub from India" , "Savoury turnover" , "Triangular Indian turnover" , "Small Indian cake of meat or vegetables" , "savoury pastry" .)