Lawful time back in the Fifties (5)
I believe the answer is:
legal
'lawful' is the definition.
(similar in meaning)
'time back in the fifties' is the wordplay.
'time' becomes 'age' (an age is a period of time).
'back' is a reversal indicator.
'in the' means one lot of letters goes inside another.
'fifties' becomes 'll' (Roman numerals).
'age' backwards is 'ega'.
'ega' put within 'll' is 'LEGAL'.
(Other definitions for legal that I've seen before include "Permissible" , "sanctioned" , "Like some tender" , "Lawful" , "Within bounds of law" .)