Flyer has left start (6)
I believe the answer is:
flinch
'start' is the definition.
(flinch is a kind of start)
'flyer has left' is the wordplay.
'flyer' becomes 'finch' (I am not sure about this - if you are sure you should believe this answer much more).
'has' is an insertion indicator.
'left' becomes 'L' (common abbreviation).
'finch' enclosing 'l' is 'FLINCH'.
Can you help me to learn more?
(Other definitions for flinch that I've seen before include "Recoil from pain, surprise etc" , "Move quickly as reaction to potential pain" , "Wince or draw back in fear" , "Start back in pain or shock" , "Shrink back with fear" .)