Lawful insertion about age in the fifties (5)
I believe the answer is:
legal
'lawful' is the definition.
(associated in meaning)
'about age in the fifties' is the wordplay.
'about' says the letters should be written backwards.
'in the' means one lot of letters goes inside another.
'fifties' becomes 'll' (Roman numerals).
'age' inserted into 'll' is 'lagel'.
'lagel' reversed gives 'LEGAL'.
'insertion' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for legal that I've seen before include "Compliant with the law" , "Relating to jurisprudence" , "Within bounds of law" , "Like some tender" , "Permitted by law" .)