Take ages to fit in (6)
I believe the answer is:
belong
'fit in' is the definition.
(to belong somewhere is to fit in there)
'take ages' is the wordplay.
'take' becomes 'be' (being is a kind of taking).
'ages' becomes 'long'.
'be'+'long'='BELONG'
'to' acts as a link.
(Other definitions for belong that I've seen before include "feel like one of the family" , "Have a proper place (in)" , "find a place?" , "Appertain" , "Be the property of" .)