Rejoin, even in age (3)
I believe the answer is:
eon
'age' is the definition.
(eon is a kind of age)
'rejoin even' is the wordplay.
'even' indicates alternate letters (letters in even-numbered positions are used).
The alternating letters of 'rejoin' are 'EON'.
'in' is the link.
(Other definitions for eon that I've seen before include "Geological period" , "Very long age" , "Very long period (US and technical spelling)" , "Vast age (alt. sp.)" , "Prolonged period" .)