Faithful servant’s last regret (4)
I believe the answer is:
true
'faithful' is the definition.
(associated in meaning)
'servant's last regret' is the wordplay.
'last' suggests the final letters.
'regret' becomes 'rue' (to rue something is to regret it).
The final letter of 'servant' is 't'.
't'+'rue'='TRUE'
(Other definitions for true that I've seen before include "to be believed?" , "indeed" , "Genuine; faithful" , "Real; accurate" , "Bona fide" .)