The kind of thread I sell specially (5)
I believe the answer is:
lisle
'the kind of thread' is the definition.
(I've seen this in another clue)
'i sell specially' is the wordplay.
'specially' indicates an anagram (I've seen this in other clues).
'i'+'sell'='isell'
'isell' is an anagram of 'LISLE'.
(Other definitions for lisle that I've seen before include "Strong cotton fabric" , "Fine smooth cotton thread for stockings" , "(Fabric woven with) a fine cotton thread" , "Twisted yarn" , "Cotton stocking thread" .)