Age in time finally, no return (3)
I believe the answer is:
eon
'age' is the definition.
(I know that eon is a type of age)
'time finally no return' is the wordplay.
'finally' says to take the final letters.
'return' shows that the letters should be reversed in order.
The final letter of 'time' is 'e'.
'no' backwards is 'on'.
'e'+'on'='EON'
'in' is the link.
(Other definitions for eon that I've seen before include "Very long age" , "Vast age (alt. sp.)" , "Vast age; eternity" , "Vast period" , "An immense time" .)