Regretting line omitted from decree (5)
I believe the answer is:
ruing
'regretting' is the definition.
('rue' can be a synonym of 'regret')
'line omitted from decree' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'omitted from' is a deletion indicator.
'decree' becomes 'ruling' (I've seen this before).
'ruling' with 'l' taken away is 'RUING'.
(Other definitions for ruing that I've seen before include "Bitterly regretting" , "feeling remorse" , "Feeling sorry for" , "Regretful" , "Having second thoughts" .)