Is sorry about the number of regulations (5)
I believe the answer is:
rules
'regulations' is the definition.
('rule' can be a synonym of 'regulation')
'is sorry about the number of' is the wordplay.
'is sorry' becomes 'rues' (I've seen this in another clue).
'about' indicates putting letters inside.
'the number of' becomes 'l' (I can't explain this - if you can you should give a lot more credence to this answer).
'rues' going around 'l' is 'RULES'.
Can you help me to learn more?
(Other definitions for rules that I've seen before include "In Australia, football" , "Conventions" , "See 7" , "Regulations, precepts" , "game in Australia" .)