A bishop in rapture tended to become forgetful (6-6)
I believe the answer is:
absent-minded
'forgetful' is the definition.
(I've seen this before)
'a bishop in rapture tended to' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'in rapture' becomes 'sent' (I've seen this in another clue).
'tended to' becomes 'minded' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'b'+'sent'+'minded'='ABSENT-MINDED'
'become' acts as a link.
Can you help me to learn more?
(Other definitions for absent-minded that I've seen before include "Scatty" , "Scatterbrained" , "Vague and forgetful" , "Prone to forgetfulness" , "Given to woolgathering" .)