Somehow nine go round the bend due to boredom (5)
I believe the answer is:
ennui
'boredom' is the definition.
(I know that boredom can be written as ennui)
'somehow nine go round the bend' is the wordplay.
'somehow' indicates an anagram.
'go round' means one lot of letters goes inside another.
'the bend' becomes 'U' (a U-bend is a kind of bend - pipe under sink).
'nine' anagrammed gives 'enni'.
'enni' enclosing 'u' is 'ENNUI'.
'due to' acts as a link.
(Other definitions for ennui that I've seen before include "Feeling when tired of life?" , "Result of boredom" , "bore" , "What's tedious and instils such listlessness" , "Boredom, listlessness" .)