Until now, hot one - hotter after erupting (8)
I believe the answer is:
hitherto
'until now' is the definition.
(I've seen this before)
'hot one hotter after erupting' is the wordplay.
'hot' becomes 'h'.
'one' becomes 'i' (Roman numeral).
'after' is a charade indicator (letters next to each other).
'erupting' indicates an anagram.
'i'+'hotter'='ihotter'
'ihotter' anagrammed gives 'itherto'.
'h'+'itherto'='HITHERTO'
(Other definitions for hitherto that I've seen before include "Up till now" , "Up to this time" , "Up to this point" , "So far" , "Heretofore (8" .)