Be-Bop-A-Lula, in part, a gem (4)
I believe the answer is:
opal
'a gem' is the definition.
(opal is a kind of gem)
'be-bop-a-lula in part' is the wordplay.
'in part' says the answer is hidden in the clue.
'OPAL' is hidden within 'bop a lula'.
(Other definitions for opal that I've seen before include "Gem-stone with a milky quality" , "Opaque mineral used a gemstone" , "Milk-coloured gemstone" , "Milky white gemstone" , "Quartz-like gemstone" .)