Sign I'm back before sunset? (5)
I believe the answer is:
minus
'sign' is the definition.
(minus sign is a kind of sign)
'i'm back before sunset?' is the wordplay.
'back' is a reversal indicator.
'before sunset?' becomes 'nus' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'im' back-to-front is 'mi'.
'mi'+'nus'='MINUS'
Can you help me to learn more?
(Other definitions for minus that I've seen before include "Lacking; below zero" , "'Take away' sign" , "Less, without" , "Reduced by the subtraction of - thus?" , "Lacking; negative" .)