Beginning to confess eating pastry (8)
I believe the answer is:
starting
'beginning' is the definition.
(I know that beginning can be written as starting)
'confess eating pastry' is the wordplay.
'confess' becomes 'sing' (sing can informally mean to talk to the police).
'eating' is an insertion indicator.
'pastry' becomes 'tart' (type of pastry).
'sing' enclosing 'tart' is 'STARTING'.
'to' acts as a link.
(Other definitions for starting that I've seen before include "showing surprise" , "Commencing" , "Disturbing" , "Rousing" , "Beginning; jumping" .)