Prompt about cover for maths book (6)
I believe the answer is:
euclid
'maths book' is the definition.
'prompt about cover' is the wordplay.
'prompt' becomes 'cue' ('cue' can be a synonym of 'prompt').
'about' says the letters should be written backwards.
'cover' becomes 'lid' (lid is a kind of cover**).
'cue' back-to-front is 'euc'.
'euc'+'lid'='EUCLID'
'for' is the link.
(Other definitions for euclid that I've seen before include "Greek father of geometry" , "Greek mathematician, d. mid-3rd century BC" , "Ancient Greek mathematician" , "6 mathematician" , "Greek philosopher" .)