Start of line crossed by bird (6)
I believe the answer is:
flinch
'start' is the definition.
(I know that flinch is a type of start)
'line crossed by bird' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'crossed by' indicates putting letters inside (inserted letters are crossed or covered by others).
'bird' becomes 'finch' (finch is a kind of bird).
'l' placed within 'finch' is 'FLINCH'.
'of' acts as a link.
(Other definitions for flinch that I've seen before include "Shrink back (out of pain/fear)" , "Draw back, with fear or pain" , "Draw back in fear, recoil" , "Draw back in shock" , "Wince or draw back in fear" .)