Full B&B has to change light source (9)
I believe the answer is:
flashbulb
'light source' is the definition.
(thesaurus)
'full bb has to change' is the wordplay.
'to change' indicates anagramming the letters.
'full'+'bb'+'has'='fullbbhas'
'fullbbhas' anagrammed gives 'FLASHBULB'.
(Other definitions for flashbulb that I've seen before include "something generating red eyes?" , "Old camera attachment" , "It may help photograph" .)