“Item of jewellery,” by the way, means “pearls” (6)
I believe the answer is:
string
'item of jewellery' is the definition.
'string' can be an answer for 'jewellery' (string is a kind of jewellery). I'm not certain of the 'item of' bit.
'by the way means pearls' is the wordplay.
'by' is a charade indicator (letters next to each other).
'the way' becomes 'st'.
'means pearls' becomes 'ring' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'st'+'ring' is 'STRING'.
Can you help me to learn more?
(Other definitions for string that I've seen before include "A lightweight cord" , "tier" , "A - - of Pearls" , "Part of racket" , "Thread; sequence" .)