Yet at last regret being faithful (4)
I believe the answer is:
true
'faithful' is the definition.
(synonyms)
'yet at last regret' is the wordplay.
'at last' suggests the final letters.
'regret' becomes 'rue' (to rue something is to regret it).
The final letter of 'yet' is 't'.
't'+'rue'='TRUE'
'being' acts as a link.
(Other definitions for true that I've seen before include "Exact; faithful" , "Loyal; authentic" , "Constant" , "Exact - loyal - honestly" , "Supported by the facts" .)