Back of missing folio should make for smoother running (3)
I believe the answer is:
oil
'should make for smoother running' is the definition.
I can't judge whether this definition defines the answer.
'back of missing folio' is the wordplay.
'back' shows that the letters should be reversed in order.
'missing' is a deletion indicator.
'folio' with 'of' taken out is 'lio'.
'lio' reversed gives 'OIL'.
Can you help me to learn more?
(Other definitions for oil that I've seen before include "Fluid lubricant" , "Slippery liquid" , "Thick liquid that doesn't dissolve in water" , "Lubricating liquid" , "Type of fuel" .)