Take long time to fit in? (6)
I believe the answer is:
belong
'to fit in?' is the definition.
(to belong somewhere is to fit in there)
'take long' is the wordplay.
'take' becomes 'be' (being is a kind of taking).
'be'+'long'='BELONG'
'time' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for belong that I've seen before include "Appertain" , "'Be part of, fit in (6)'" , "Be rightly assigned (to)" , "To be entirely suitable (to place)" , "Be owned by" .)