Container in lava tubes (3)
I believe the answer is:
vat
'container' is the definition.
(vat is a kind of container)
'in lava tubes' is the wordplay.
'in' says the answer is hidden in the clue.
'VAT' is hidden within 'lava tubes'.
(Other definitions for vat that I've seen before include "Large tank; purchase tax" , "A tax on a large container?" , "Liquid storage receptacle" , "Large tub; tax" , "Barrel - tax" .)