One could mount it with saintly mien (5)
I believe the answer is:
stair
'one could mount' is the definition.
I can't tell whether this definition defines the answer.
'it with saintly mien' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'with' is a charade indicator (letters next to each other).
'saintly' becomes 's' (this could be a standard abbreviation I've not previously seen).
'mien' becomes 'air' (I've seen this before).
't' after 's' is 'st'.
'st'+'air'='STAIR'
Can you help me to learn more?
(Other definitions for stair that I've seen before include "means to go down" , "Bit of a flight!" , "single step in a series" , "one on the way up" , "Tread and riser" .)