Take ages to get fit? (6)
I believe the answer is:
belong
'fit?' is the definition.
(thesaurus)
'take ages' is the wordplay.
'take' becomes 'be' (being is a kind of taking).
'ages' becomes 'long'.
'be'+'long'='BELONG'
'to get' is the link.
(Other definitions for belong that I've seen before include "Be the property of" , "Appertain" , "Be rightly assigned (to)" , "Pertain" , "accepted socially" .)