Pure class to be straddling horse (6)
I believe the answer is:
chaste
'pure' is the definition.
(chaste means abstaining from sex)
'class to be straddling horse' is the wordplay.
'class' becomes 'caste' (type of social class).
'to be straddling' is an insertion indicator (straddle can mean to lie on both sides of).
'horse' becomes 'h' (synonyms).
'caste' enclosing 'h' is 'CHASTE'.
(Other definitions for chaste that I've seen before include "Innocent" , "Refraining from sex" , "Continent" , "Cheats (anag.)" , "Pure and decent" .)