Constituent of popular Tory leader with substantial backing (8)
I believe the answer is:
integral
'constituent' is the definition.
(I've seen this before)
'popular tory leader with substantial backing' is the wordplay.
'popular' becomes 'in' (something that's 'in' is fashionable).
'leader' suggests taking the first letters.
'with' says to put letters next to each other.
'substantial' becomes 'large' (substantial can mean large or considerable).
'backing' is a reversal indicator.
The initial letter of 'tory' is 't'.
'large' reversed gives 'egral'.
'in'+'t'+'egral'='INTEGRAL'
'of' acts as a link.
(Other definitions for integral that I've seen before include "Formed of constituent parts" , "Belonging as an essential part" , "Fundamental; function" , "Triangle (anag)" , "Unimpaired" .)