Ian and Luke, wildly different (7)
I believe the answer is:
unalike
'different' is the definition.
(I've seen this in another clue)
'ian and luke wildly' is the wordplay.
'and' means one lot of letters go next to another.
'wildly' is an anagram indicator.
'ian' after 'luke' is 'lukeian'.
'lukeian' anagrammed gives 'UNALIKE'.
(Other definitions for unalike that I've seen before include "poor match" , "different" , "Not similar to one another" , "Dissimilar" , "Incompatible" .)