To say more in French is, after a touch of Spanish, most regrettable (7)
I believe the answer is:
saddest
'most regrettable' is the definition.
(sad can mean regrettable)
'to say more in french is after a touch of spanish' is the wordplay.
'to say more' becomes 'add' (add can mean to say something else).
'in' indicates putting letters inside.
'french is' becomes 'est' ('is' in French).
'after' says to put letters next to each other.
'a touch of' indicates taking the first letters.
The first letter of 'spanish' is 's'.
'est' put after 's' is 'sest'.
'add' inserted within 'sest' is 'SADDEST'.
(Other definitions for saddest that I've seen before include "Most doleful" , "Unhappiest" , "Most blue" , "Most gloomy" , "Most unhappy" .)