I see a worm turn when it is tedious (9)
I believe the answer is:
wearisome
'it is tedious' is the definition.
('wearisome' can be a synonym of 'tedious')
'i see a worm turn' is the wordplay.
'turn' indicates anagramming the letters.
'i'+'see'+'a'+'worm'='iseeaworm'
'iseeaworm' with letters rearranged gives 'WEARISOME'.
'when' is the link.
(Other definitions for wearisome that I've seen before include "Vexatious" , "pestilential" , "Boring" , "Tedious, tiring" , "Trying" .)