Long, ultimately dire boring thread (5)
I believe the answer is:
yearn
'long' is the definition.
(I know that long can be written as yearn)
'ultimately dire boring thread' is the wordplay.
'ultimately dire' becomes 'e' (I can't justify this - if you can you should give a lot more credence to this answer).
'boring' means one lot of letters goes inside another (some letters must 'bore' a hole into other letters).
'thread' becomes 'yarn' (synonyms).
'e' inserted into 'yarn' is 'YEARN'.
Can you help me to learn more?
(Other definitions for yearn that I've seen before include "Have a craving" , "Long - pine" , "Mark of injury" , "Have an intense feeling of longing" , "Ache, crave" .)