Betty regularly keeps card up sleeve (6)
I believe the answer is:
jacket
'betty regularly' is the definition.
I can't judge whether this definition defines the answer.
'card up sleeve' is the wordplay.
'card' becomes 'jack' (type of playing card).
'up sleeve' becomes 'et' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'jack'+'et'='JACKET'
'keeps' acts as a link.
Can you help me to learn more?
(Other definitions for jacket that I've seen before include "See 1" , "Outer skin of a potato" , "Short hip-length coat" , "Record sleeve" , "Outer garment or casing" .)