A number badly regret their occupancy (6)
I believe the answer is:
tenure
'occupancy' is the definition.
(I've seen this before)
'a number badly' is the wordplay.
'a number' becomes 'ten' (ten is a number).
'badly' becomes 'ure' (I can't explain this - if you can you should give a lot more credence to this answer).
'ten'+'ure'='TENURE'
'regret their' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tenure that I've seen before include "(Right of) occupancy" , "Right or period to hold, university post say" , "Permanent employment right" , "The holding of office or land" , "Neuter (anag)" .)