No turning back: determined to make start (5)
I believe the answer is:
onset
'start' is the definition.
(onset is a kind of start)
'no turning back determined' is the wordplay.
'turning back' shows that the letters should be reversed in order.
'determined' becomes 'set' (similar in meaning).
'no' in reverse letter order is 'on'.
'on'+'set'='ONSET'
'to make' acts as a link.
(Other definitions for onset that I've seen before include "Assault; start" , "First signs" , "Beginning; military attack" , "Start - opening moments" , "Outbreak" .)