Succeed in being extremely unshakeable (5)
I believe the answer is:
ensue
'succeed' is the definition.
(I've seen this in another clue)
'being extremely unshakeable' is the wordplay.
'being' becomes 'ens' (philosophical term for being or existence).
'extremely' suggests removing the centre (letters on the extremes of the word are used).
'unshakeable' with its middle taken out is 'ue'.
'ens'+'ue'='ENSUE'
'in' acts as a link.
(Other definitions for ensue that I've seen before include "Proceed" , "Arise" , "Follow afterwards" , "Happen or come about as consequence" , "Come about, result in" .)