King won over signora's heart -- that's understood (5)
I believe the answer is:
known
'understood' is the definition.
('known' can be similar in meaning to 'understood')
'king won over signora's heart' is the wordplay.
'king' becomes 'k'.
'over' is a reversal indicator.
'heart' means to look at the middle letters.
The centre of 'signora' is 'n'.
'won' reversed gives 'now'.
'k'+'now'+'n'='KNOWN'
'that's' is the link.
(Other definitions for known that I've seen before include "In the public domain?" , "Recognised, familiar" , "Noted" , "Apprehended mentally" , "Widely recognised" .)