Go back inside serene geyser (6)
I believe the answer is:
renege
'go back' is the definition.
(I've seen this in another clue)
'inside serene geyser' is the wordplay.
'inside' says the answer is hidden in the clue.
'RENEGE' is hidden within 'serene geyser'.
(Other definitions for renege that I've seen before include "Go back (on promise etc)" , "Fail to follow suit or fulfil a promise" , "Fail to honour an obligation" , "Fail to fulfil a promise or obligation" , "South American cowboy" .)