Extremely fickle twin sons own up (4)
I believe the answer is:
fess
'own up' is the definition.
(confess)
'extremely fickle twin sons' is the wordplay.
'twin sons' means two replacements for 'sons'.
'extremely' suggests removing the centre (only the extremes of the word are used).
'sons' becomes 's'.
'sons' becomes 's'.
'fickle' with its centre removed is 'fe'.
'fe'+'s'+'s'='FESS'
(Other definitions for fess that I've seen before include "Ordinary" , "Own up to crime" .)