Regretted having left the last word initially to editor (8)
I believe the answer is:
lamented
'regretted' is the definition.
('lament' can be a synonym of 'regret')
'left the last word initially to editor' is the wordplay.
'left' becomes 'l' (common abbreviation**).
'the last word' becomes 'amen' (I've seen this before).
'initially' says to take the initial letters.
'editor' becomes 'ed'.
The first letter of 'to' is 't'.
'l'+'amen'+'t'+'ed'='LAMENTED'
'having' is the link.
(Other definitions for lamented that I've seen before include "Mourned (a death)" , "Mourned or grieved for" , "Bemoaned" , "Non compos mentis" , "Expressed disappointment about (something)" .)