Worry frequently with French and ... (4)
I believe the answer is:
fret
'worry' is the definition.
(to fret about something is to worry)
'frequently with french and' is the wordplay.
'frequently' becomes 'fr' (abbreviation).
'with' says to put letters next to each other.
'french and' becomes 'et' ('and' in French).
'fr'+'et'='FRET'
(Other definitions for fret that I've seen before include "worry its way through?" , "Worry - decoration - part of guitar" , "Worry; or the marking on a guitar" , "Display anxiety" , "Anguish" .)