Copy needs taking out for checking age (6,6)
I believe the answer is:
carbon dating
'checking age' is the definition.
(means of checking how old something is)
'copy needs taking out' is the wordplay.
'copy' becomes 'carbon' (as in a carbon copy).
'needs' says to put letters next to each other.
'taking out' becomes 'dating' (taking on dates).
'carbon'+'dating'='CARBON DATING'
'for' is the link.
(Other definitions for carbon dating that I've seen before include "Method for determining the age of old matter" , "Means of finding age of prehistoric items" , "ageing process" , "scientific process" , "The use of isotopes to determine age" .)