Finish union trouble outside with insinuation (8)
I believe the answer is:
innuendo
'insinuation' is the definition.
('innuendo' can be a synonym of 'insinuation')
'finish union trouble outside' is the wordplay.
'finish' becomes 'end' ('end' can be a synonym of 'finish'**).
'trouble' indicates an anagram.
'outside' indicates putting letters inside.
'union' is an anagram of 'innuo'.
'end' going within 'innuo' is 'INNUENDO'.
'with' is the link.
(Other definitions for innuendo that I've seen before include "An unpleasant insinuation or intimation" , "Subtle disparagement" , "Undone in unpleasant insinuation" , "Suggestive allusion" , "Suggestiveness" .)