Left going in terribly straight to produce shiner (9)
I believe the answer is:
starlight
'shiner' is the definition.
(something which shines)
'left going in terribly straight' is the wordplay.
'left' becomes 'l' (common abbreviation).
'going in' is an insertion indicator.
'terribly' is an anagram indicator.
'straight' is an anagram of 'staright'.
'l' placed into 'staright' is 'STARLIGHT'.
'to produce' is the link.
(Other definitions for starlight that I've seen before include "The illumination of a moonless night" , "distant illumination - with 7d" , "faint illumination" , "Twinkling overhead" , "faint glimmer" .)