Set containing a pearl (4)
I believe the answer is:
bead
'pearl' is the definition.
(I know that pearl can be written as bead)
'set containing a' is the wordplay.
'set' becomes 'bed' (bedding is a kind of setting).
'containing' indicates putting letters inside.
'bed' enclosing 'a' is 'BEAD'.
(Other definitions for bead that I've seen before include "Part of a necklace" , "Necklace component" , "Necklace element" , "Element of rosary" , "Spot of dew; one threaded on necklace" .)