Jingle by setter, say “midnight before long” (8)
I believe the answer is:
'jingle' is the definition.
'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'.
'by' acts as a link.
(Other definitions for doggerel that I've seen before include "Rubbishy verse", "Poor quality comic or trivial verse", "lines of inferior quality", "Burlesque verse", "not great poetry".)