It's plain it has no stairs to climb (4)
I believe the answer is:
flat
'it's plain' is the definition.
(flat is a kind of plain)
'it has no stairs to climb' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'has' is a charade indicator (letters next to each other).
'no stairs' becomes 'alf' (I can't explain this - if you can you should give a lot more credence to this answer).
'to climb' says the letters should be written backwards (I've seen 'climbs' mean this).
't'+'alf'='talf'
'talf' written backwards gives 'FLAT'.
Can you help me to learn more?
(Other definitions for flat that I've seen before include "Pied-a-terre" , "Small residence - lacking sparkle" , "Level - uninteresting - accommodation" , "Personal residence" , "unexciting" .)