Fortunately core members prefer to be different (7)
I believe the answer is:
unalike
'different' is the definition.
(I've seen this before)
'fortunately core members' is the wordplay.
'core' means to look at the middle letters.
'members' becomes 'like' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The centre of 'fortunately' is 'una'.
'una'+'like'='UNALIKE'
'prefer to be' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for unalike that I've seen before include "different" , "Dissimilar" , "Incompatible" , "Differing from each other" , "Not similar to one another" .)