Ghost once seen in cellar vault (5)
I believe the answer is:
larva
'ghost' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for larva that I've seen before include "Caterpillar perhaps" , "Grub; insect" , "Worm-like insect before metamorphosis" , "Insect's immature form" , "Early stage of creature" .)