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 before).
'about' means one lot of letters goes inside another.
'the number of' becomes 'l' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'rues' enclosing '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" .)