Overcome in Lourdes, one supporting saint (4)
I believe the answer is:
stun
'overcome' is the definition.
(thesaurus)
'in lourdes one supporting saint' is the wordplay.
'in lourdes one' becomes 'un' ('one' in French, eg in Lourdes).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'saint' becomes 'st'.
'un' put after 'st' is 'STUN'.
(Other definitions for stun that I've seen before include "Render insensitive" , "Amaze; knock senseless" , "Render temporarily senseless" , "Make senseless or dizzy with a blow" , "Daze, overwhelm" .)