Points out one looking sickly (3)
I believe the answer is:
wan
'looking sickly' is the definition.
(I've seen this in another clue)
'points out one' is the wordplay.
'points' becomes 'wn' ().
'out' means one lot of letters goes inside another.
'one' becomes 'a' (a thing is one thing).
'wn' enclosing 'a' is 'WAN'.
(Other definitions for wan that I've seen before include "Pale, looking exhausted" , "Faint" , "Direly pale" , "Pasty" , "Pale as from illness" .)