Don't forget about one who's in the club (8)
I believe the answer is:
remember
'don't forget' is the definition.
(I've seen this before)
'about one who's in the club' is the wordplay.
'about' becomes 're' (regarding).
'one who's in the club' becomes 'member' (I am not sure about this - if you are sure you should believe this answer much more).
're'+'member'='REMEMBER'
Can you help me to learn more?
(Other definitions for remember that I've seen before include "Don't forget" , "Recal, recollect" , "Keep in mind" , "Mark" , "Commemorate" .)