Turns up, source of illumination about first race is produced as publicity for the bookmaker (5)
I believe the answer is:
Here is my best explanation:
'produced as publicity for the bookmaker' is the definition.
I can't judge whether this defines the answer.
'turns up source of illumination about first race' is the wordplay.
'turns up' says the letters should be written in reverse.
'source of illumination' becomes 'bulb' (bulb is a kind of source of illumination).
'about' is an insertion indicator.
'first' says to take the initial letters.
The initial letter of 'race' is 'r'.
'bulb' enclosing 'r' is 'brulb'.
'brulb' backwards is 'BLURB'.
'is' acts as a link.
(Other definitions for blurb that I've seen before include "Advertising text", "Publisher's descrtion of book", "Puff", "Publisher's text on book", "Promotional statement, on book-cover say", "Short description on book cover", "Advertising description praising something", "printed on jacket", "Book jacket commendation" and "Publisher's promotional description".)