Regret theorem — or sentiment framing it (7)
I believe the answer is:
remorse
'regret theorem' is the definition.
'remorse' can be an answer for 'regret' (remorse is a feeling of regret). I am not sure about the 'theorem' bit.
'or sentiment framing it' is the wordplay.
'sentiment' becomes 'rems' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'framing' is an insertion indicator (inserted letters are framed by the outside letters).
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'or' inserted into 'rems' is 'remors'.
'remors'+'e'='REMORSE'
Can you help me to learn more?
(Other definitions for remorse that I've seen before include "Regret for wrong" , "it may bite" , "Sorrow for wrong committed" , "regrets" , "Bitter regret" .)