Is king having regrets about heading for London? (5)
I believe the answer is:
rules
'is king' is the definition.
(I've seen this in another clue)
'regrets about heading for london?' is the wordplay.
'regrets' becomes 'rues' ('rue' can be a synonym of 'regret').
'about' is an insertion indicator.
'heading for' suggests taking the first letters.
The first letter of 'london' is 'l'.
'rues' placed around 'l' is 'RULES'.
'having' is the link.
(Other definitions for rules that I've seen before include "Australian football" , "Is king" , "Reigns" , "game in Australia" , "See 7" .)