Compel former pupil to be dishonest about key (6)
I believe the answer is:
oblige
'compel' is the definition.
(I know that compel can be written as oblige)
'former pupil to be dishonest about key' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy').
'to be dishonest' becomes 'gil' (I can't justify this - if you can you should believe this answer much more).
'about' is a reversal indicator.
'key' becomes 'E' (a musical key).
'gil' backwards is 'lig'.
'ob'+'lig'+'e'='OBLIGE'
Can you help me to learn more?
(Other definitions for oblige that I've seen before include "Perform a favour for" , "Hold in debt" , "Compel, bind" , "Make" , "Compel legally, morally" .)