Regretting governing without first law (5)
I believe the answer is:
ruing
'regretting' is the definition.
('rue' can be a synonym of 'regret')
'governing without first law' is the wordplay.
'governing' becomes 'ruling' ('rule' can be a synonym of 'govern').
'without' suggests deleting specific letters.
'first law' becomes 'l' (1st letter of 'law').
'ruling' with 'l' removed is 'RUING'.
(Other definitions for ruing that I've seen before include "Bitterly regretting" , "feeling remorse" , "Regretful" , "Having second thoughts" , "full of remorse?" .)