Thread I sell knotted! (5)
I believe the answer is:
lisle
'thread' is the definition.
(lisle is a kind of thread)
'i sell knotted' is the wordplay.
'knotted' indicates anagramming the letters.
'i'+'sell'='isell'
'isell' anagrammed gives 'LISLE'.
(Other definitions for lisle that I've seen before include "Strong cotton material" , "(Fabric woven with) a fine cotton thread" , "Maker of hose" , "Strong tightly twisted cotton thread" , "Fine cotton thread used in stockings" .)