Pearls are round, by the way (6)
I believe the answer is:
string
'pearls are' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'round by the way' is the wordplay.
'round' becomes 'ring' ('ring' can be a synonym of 'round'**).
'by' means one lot of letters go next to another.
'the way' becomes 'st'.
'ring' put after 'st' is 'STRING'.
Can you help me to learn more?
(Other definitions for string that I've seen before include "Narrow cord or kind of mesh vest" , "Thin length of cord etc" , "Succession" , "This vest is made of large mesh" , "Sequence; cord" .)