Start a dismissed firm (6)
I believe the answer is:
outset
'start' is the definition.
(I know that start can be written as outset)
'a dismissed firm' is the wordplay.
'a dismissed' becomes 'out' (I've seen this before).
'firm' becomes 'set' (both can mean fixed).
'out'+'set'='OUTSET'
(Other definitions for outset that I've seen before include "Set out in the beginning" , "launch" , "debut?" , "origin" , "Starting point" .)