Is of use when lava is erupting (6)
I believe the answer is:
avails
'is of use' is the definition.
(I've seen this before)
'lava is erupting' is the wordplay.
'erupting' is an anagram indicator.
'lava'+'is'='lavais'
'lavais' anagrammed gives 'AVAILS'.
'when' is the link.
(Other definitions for avails that I've seen before include "Is of profit, helps" , "Takes advantage of or benefits from" , "Makes use of saliva" .)