Former criminal (90) is inside a long time (2-3)
I believe the answer is:
ex-con
'former criminal' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'90 is inside a long time' is the wordplay.
'90' becomes 'xc' (Roman numerals).
'is inside' indicates putting letters inside.
'a long time' becomes 'eon' (eon is a kind of long time).
'xc' going inside 'eon' is 'EX-CON'.
Can you help me to learn more?
(Other definitions for ex-con that I've seen before include "Person once inside, now outside" , "She used to serve porridge" , "Former jailbird" , "having defected from Tory party?" , "Former prisoner, in short" .)