Sickly-looking magician’s prop incomplete (3)
I believe the answer is:
wan
'sickly-looking' is the definition.
'magician's prop incomplete' is the wordplay.
'magician's prop' becomes 'wand' (I've seen this before).
'incomplete' means to remove the last letter.
'wand' with its final letter taken off is 'WAN'.
(Other definitions for wan that I've seen before include "Blanched" , "Pale from sickness" , "Pallid" , "Pale and weak-looking" , "Pale in Botswana" .)