Begin to have a meal, consuming flan (5,2)
I believe the answer is:
start up
'begin' is the definition.
('start up' can be a synonym of 'begin')
'to have a meal consuming flan' is the wordplay.
'to have a meal' becomes 'sup'.
'consuming' is an insertion indicator.
'flan' becomes 'tart' (I've seen this before).
'sup' placed around 'tart' is 'START-UP'.
(Other definitions for start up that I've seen before include "Found" , "Beginning" , "Begin" , "Newly-established firm" , "newly formed company" .)