Friend kept back after school scripture lesson: it increases pressure (10)
I believe the answer is:
compressor
'it increases pressure' is the definition.
The definition suggests a singular noun which matches the answer.
'friend kept back after school scripture lesson' is the wordplay.
'friend' becomes 'ross' (I can't explain this - if you can you should give a lot more credence to this answer).
'kept back' shows that the letters should be reversed in order.
'after' means one lot of letters go next to another.
'school' becomes 'comp' (short for comprehensive school).
'scripture lesson' becomes 'RE' (abbreviation for Religious Education).
'ross' in reverse letter order is 'ssor'.
'comp'+'re'='compre'
'ssor' put after 'compre' is 'COMPRESSOR'.
Can you help me to learn more?
(Another definition for compressor that I've seen is " Muscle".)