Worry about moving into empty flat (4)
I believe the answer is:
fret
'worry' is the definition.
(to fret about something is to worry)
'about moving into empty flat' is the wordplay.
'about' becomes 're' (regarding).
'moving into' means one lot of letters goes inside another.
'empty' says to hollow out the word (remove centre letters).
'flat' with its middle taken out is 'ft'.
're' placed inside 'ft' is 'FRET'.
(Other definitions for fret that I've seen before include "Chafe; worry" , "Worry - pine" , "Show anxiety" , "Wear away" , "ridge > that obscures sea view" .)