Inclination reading revised introduction to text (8)
I believe the answer is:
gradient
'inclination' is the definition.
(gradient is a kind of inclination)
'reading revised introduction to text' is the wordplay.
'revised' is an anagram indicator.
'introduction to' indicates taking the first letters (I've seen 'introductions to' mean this).
The initial letter of 'text' is 't'.
'reading' anagrammed gives 'gradien'.
'gradien'+'t'='GRADIENT'
(Other definitions for gradient that I've seen before include "Measure of slope" , "Rise" , "Treading (anag) - slope" , "Inclination" , "Angle of slope" .)