Bit of pastry on outside of cake (5)
I believe the answer is:
piece
'bit of' is the definition.
(I've seen this before)
'pastry on outside of cake' is the wordplay.
'pastry' becomes 'pie' (pie is a kind of pastry).
'on' is a charade indicator (letters next to each other).
'outside of' suggests removing the centre.
'cake' with its middle taken out is 'ce'.
'pie'+'ce'='PIECE'
(Other definitions for piece that I've seen before include "Component; portion" , "wedge a > bit" , "Separate part of a whole" , "One part of a whole" , "Slice" .)