At first, Etna’s rumbling, under pressure to — this? (5)
I believe the answer is:
erupt
'this?' is the definition.
The definition and answer are not the same part of speech.
'at first etna's rumbling under pressure to' is the wordplay.
'at first' says to take the initial letters.
The initial letters of 'etna rumbling under pressure to' is 'ERUPT'.
(Other definitions for erupt that I've seen before include "Suddenly appear" , "be active" , "Burst forth, like 25" , "suddenly burst out" , "Discharge" .)