Start getting leg fixed (5)
I believe the answer is:
onset
'start' is the definition.
(both can mean the beginning of something)
'leg fixed' is the wordplay.
'leg' becomes 'on' (cricket term, fielding positions).
'fixed' becomes 'set' (both can mean decided or determined).
'on'+'set'='ONSET'
'getting' acts as a link.
(Other definitions for onset that I've seen before include "Assault; start" , "Beginning or early stages" , "Start, beginning" , "First signs" , "Beginning (of something often unpleasant)" .)