Victory over old worry (3)
I believe the answer is:
vex
'worry' is the definition.
(I know that worry can be written as vex)
'victory over old' is the wordplay.
'victory' becomes 'v'.
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'old' becomes 'ex' (prefix meaning former or onetime).
'v'+'ex'='VEX'
(Other definitions for vex that I've seen before include "upset" , "Irritate, annoy" , "Irk" .)