Worry about breaking foot (4)
I believe the answer is:
fret
'worry' is the definition.
(to fret about something is to worry)
'about breaking foot' is the wordplay.
'about' becomes 're' (regarding).
'breaking' means one lot of letters goes inside another.
'foot' becomes 'ft' (abbreviation).
're' put inside 'ft' is 'FRET'.
(Other definitions for fret that I've seen before include "Worry; wet mist" , "worry its way through?" , "Show anxiety" , "Split up" , "Wear away" .)