Former lord crushed by one’s first spell in jail (3-4)
I believe the answer is:
old-time
'former' is the definition.
'lord crushed by one's first spell in jail' is the wordplay.
'lord' becomes 'ld' (abbreviation for the aristocratic title).
'crushed by' is an insertion indicator.
'first' suggests taking the first letters.
'spell in jail' becomes 'time' (someone in jail is 'doing time').
The first letter of 'ones' is 'o'.
'o'+'time'='otime'
'ld' placed into 'otime' is 'OLD-TIME'.
(Other definitions for old-time that I've seen before include "(E.g. dancing) in ancient style" , "Relating to the past" , "Quaint" .)