Initial tenor to regret being faithful (4)
I believe the answer is:
true
'faithful' is the definition.
(associated in meaning)
'initial tenor to regret' is the wordplay.
'initial' says to take the initial letters.
'to regret' becomes 'rue' (to rue something is to regret it).
The first letter of 'tenor' is 't'.
't'+'rue'='TRUE'
'being' acts as a link.
(Other definitions for true that I've seen before include "Straight or correct" , "Honest - expression of agreement" , "indeed" , "Factually correct" , "Exact, factual" .)