Tom overtly has to budge (4)
I believe the answer is:
move
'to budge' is the definition.
(to budge is to move slightly)
'tom overtly has' is the wordplay.
'has' indicates a hidden word.
'MOVE' is hidden in the letters of 'tom overtly'.
(Other definitions for move that I've seen before include "Depart" , "Change (house)" , "change position on board" , "Change location" , "Manoeuvre; go" .)