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' is a reversal indicator.
'in the' means one lot of letters goes inside another.
'fifties' becomes 'll' (the Roman numeral for 50 written twice).
'age' placed into 'll' is 'lagel'.
'lagel' backwards is 'LEGAL'.
'insertion' is the 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" .)