Lacking introduction, fix conclusion (3)
I believe the answer is:
end
'conclusion' is the definition.
(something's conclusion is its end)
'lacking introduction fix' is the wordplay.
'lacking introduction' means to remove the first letter.
'fix' becomes 'mend' (synonyms).
'mend' with its initial letter taken off is 'END'.
(Other definitions for end that I've seen before include "result" , "Finished" , "For Chelsea or Villa, it's a" , "Limit" , "ambition" .)