Thread is line in middle of bulletin (5)
I believe the answer is:
lisle
'thread' is the definition.
(I know that lisle is a type of cotton)
'is line in middle of bulletin' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'in' indicates putting letters inside.
'middle of' says to take the centre.
The centre of 'bulletin' is 'le'.
'is'+'l'='isl'
'isl' put within 'le' is 'LISLE'.
(Other definitions for lisle that I've seen before include "Strong cotton fabric" , "Twisted yarn" , "Strong tightly twisted cotton thread" , "Cotton stocking thread" , "Strong cotton material" .)