On the surface (5)
(Other definitions for outer that I've seen before include "Ring for double scores on a dartboard" , "Poor shot" , "On the periphery" , "Exterior or external, like space" , "Away from the centre" .)
'on the surface' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for superficial that I've seen before include "12 [TWO DIMENSIONAL]" , "Not profound" , "Far from thorough" , "Cursory" .)