Regrets accommodating fifty guides (5)
I believe the answer is:
rules
'guides' is the definition.
(as in guidelines)
'regrets accommodating fifty' is the wordplay.
'regrets' becomes 'rues' ('rue' can be a synonym of 'regret').
'accommodating' means one lot of letters goes inside another.
'fifty' becomes 'L' (Roman numeral).
'rues' going around 'l' is 'RULES'.
(Other definitions for rules that I've seen before include "game in Australia" , "Reigns" , "See 7" , "In Australia, football" , "Laws; draws lines" .)