Jingle by setter, say “midnight before long” (8)
I believe the answer is:
doggerel
'jingle' is the definition.
(synonyms)
'setter say midnight before long' is the wordplay.
'setter say' becomes 'dog' (setter is a type of dog).
'midnight' becomes 'g' (the middle letter in 'night').
'before' becomes 'ere' (poetic term for 'before').
'long' becomes 'l'.
'dog'+'g'+'ere'+'l'='DOGGEREL'
'by' acts as a link.
(Other definitions for doggerel that I've seen before include "Badly written verse" , "Very bad poetry or verse" , "trivial lines" , "lines of inferior quality" , "Burlesque verse" .)