Requirement with second half of tale - one mustn't lose the thread (6)
I believe the answer is:
needle
'tale one mustn't lose the thread' is the definition.
I can't tell whether this defines the answer.
'requirement with second half' is the wordplay.
'requirement' becomes 'need' ('need' can be a synonym of 'requirement').
'with second half' becomes 'le' (I can't explain this - if you can you should give a lot more credence to this answer).
'need'+'le'='NEEDLE'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for needle that I've seen before include "Enmity" , "Provocation" , "Pointer - irritate" , "Implement with eye" , "Annoy by constant criticism" .)