President's vow in ceremony for all to see (8)
I believe the answer is:
pompidou
'president's' is the definition.
(French president Georges Pompidou)
'vow in ceremony for all to see' is the wordplay.
'vow' becomes 'I do' (I do - vow said at weddings).
'in' means one lot of letters goes inside another.
'ceremony' becomes 'pomp' (I've seen this in another clue).
'for all to see' becomes 'u' (short for Universal film certificate).
'pomp'+'u'='pompu'
'ido' placed within 'pompu' is 'POMPIDOU'.
(Other definitions for pompidou that I've seen before include "statesman" , "Once leading Frenchman" , "Paris arts centre name" .)