A regret, without good reason (5)
I believe the answer is:
argue
'reason' is the definition.
(synonyms)
'a regret without good' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'without' means one lot of letters goes inside another ('without' can be similar in meaning to 'outside').
'good' becomes 'g' (abbreviation).
'rue' placed around 'g' is 'rgue'.
'a'+'rgue'='ARGUE'
(Other definitions for argue that I've seen before include "Dispute, disagree" , "Fall out" , "Have a difference of opinion" , "Debate or disagree" , "Dispute heatedly" .)