Volcano initially falling under Jovian influence (4)
I believe the answer is:
fuji
'volcano' is the definition.
(I have seen 'Japanese volcano ' mean 'fuji' so perhaps 'volcano' could also mean 'fuji')
'initially falling under jovian influence' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'falling under jovian influence' is 'FUJI'.
(Other definitions for fuji that I've seen before include "Extinct Japanese volcano - cherry tree" , "Extinct volcano in central Japan" , "Highest Jap. mountain" , "Extinct volcano in south central Honshu" , "Japan's highest peak" .)