That woman's back, extremely sensible - why most people don't believe this! (6)
I believe the answer is:
heresy
'most people don't believe this' is the definition.
The definition suggests a singular noun which matches the answer.
'that woman's back extremely sensible why' is the wordplay.
'that woman's' becomes 'her' (I've seen this before).
'back' shows that the letters should be reversed in order.
'extremely' means to remove the middle letters.
'why' becomes 'y'.
'sensible' with its middle removed is 'se'.
'se' in reverse letter order is 'es'.
'her'+'es'+'y'='HERESY'
Can you help me to learn more?
(Other definitions for heresy that I've seen before include "Opinion contrary to those of orthodox religion" , "Unorthodox view" , "Religious non-conformity" , "Doctrine at variance with the orthodox position" , "Unorthodox religious opinion" .)