Not in fixed start (6)
I believe the answer is:
outset
'start' is the definition.
(I know that start can be written as outset)
'not in fixed' is the wordplay.
'not in' becomes 'out' ('out' is the opposite to 'in').
'fixed' becomes 'set' (both can mean decided or determined).
'out'+'set'='OUTSET'
(Other definitions for outset that I've seen before include "Kickoff time?" , "Fruit" , "Starting point" , "debut?" , "Set out in the beginning" .)