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' is an insertion indicator.
'the number of' becomes 'l' (I can't justify this - if you can 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" .)