Mark Zuckerberg ultimately engaged in crime (4)
I believe the answer is:
sign
'mark' is the definition.
(I know that mark can be written as sign)
'zuckerberg ultimately engaged in crime' is the wordplay.
'ultimately' suggests the final letters.
'engaged in' is an insertion indicator.
'crime' becomes 'sin'.
The final letter of 'zuckerberg' is 'g'.
'g' inserted within 'sin' is 'SIGN'.
(Other definitions for sign that I've seen before include "Leo, for instance" , "Traffic control - to write one's name" , "part of the zodiac" , "Indication - put one's name down" , "Show" .)