Persistent criminal caught by lying occasionally (8)
I believe the answer is:
lifelong
'persistent' is the definition.
'criminal caught by lying occasionally' is the wordplay.
'criminal' becomes 'felon' ('felon' can be a synonym of 'criminal'**).
'caught by' is an insertion indicator.
'occasionally' means one should take alternating letters.
The alternate letters of 'lying' are 'lig'.
'felon' inserted into 'lig' is 'LIFELONG'.
(Other definitions for lifelong that I've seen before include "Till death" , "Birth-to-death" , "with no possibility of release" , "Lasting from birth to death" , "serious custodial sentence" .)