One mounted with saintly mien? (5)
I believe the answer is:
stair
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'mounted with saintly mien?' is the wordplay.
'mounted' says the letters should be written backwards.
'with saintly' becomes 'ts' (I can't justify this - if you can you should believe this answer much more).
'mien?' becomes 'air' (I've seen this before).
'ts' written backwards gives 'st'.
'st'+'air'='STAIR'
Can you help me to learn more?
(Other definitions for stair that I've seen before include "Flight unit" , "One of a 15" , "Step as part of flight" , "One step of a flight" , "Tread and riser" .)