Karen joining odyssey travelling for a long time (7,5)
I believe the answer is:
donkeys years
'long time' is the definition.
(I've seen this before)
'karen joining odyssey travelling' is the wordplay.
'joining' means one lot of letters go next to another.
'travelling' indicates an anagram.
'karen'+'odyssey'='karenodyssey'
'karenodyssey' is an anagram of 'DONKEYS YEARS'.
'for a' is the link.
(Other definitions for donkeys years that I've seen before include "An age" , "A long time (spent on Blackpool beach?)" , "A beastly long time" , "What's evident in old ass" , "Ages" .)