Regret bringing in Labour's first law (4)
I believe the answer is:
rule
'law' is the definition.
(I know that rule is a type of law)
'regret bringing in labour's' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'bringing in' means one lot of letters goes inside another.
'labour' becomes 'L' (political abbreviation).
'rue' going around 'l' is 'RULE'.
'first' acts as a link.
(Other definitions for rule that I've seen before include "Decree" , "Dominion" , "Control, government" , "Standard" , "Exercise power (over)" .)