Sign on back of door, only reversed on the outside (5)
I believe the answer is:
enrol
'sign on' is the definition.
(I've seen this before)
'back of door only reversed on the outside' is the wordplay.
'back of door' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'only' becomes 'lone' (synonyms).
'reversed' is a reversal indicator.
'on the outside' indicates putting letters inside.
'lone' written backwards gives 'enol'.
'r' going within 'enol' is 'ENROL'.
Can you help me to learn more?
(Other definitions for enrol that I've seen before include "Join as member" , "Recruit" , "Sign up (member)" , "Insert in register" , "Register (as member)" .)