Most pleasant since breaking up with model (6)
I believe the answer is:
nicest
'most pleasant' is the definition.
('nice' can be similar in meaning to 'pleasant')
'since breaking up with model' is the wordplay.
'breaking up' is an anagram indicator.
'with' means one lot of letters go next to another.
'model' becomes 't' (Ford Model T car).
'since' anagrammed gives 'nices'.
'nices'+'t'='NICEST'
(Other definitions for nicest that I've seen before include "Insect (anag)" , "I scent in the most pleasant way" , "Most appealing" , "Most agreeable" , "Sin etc (anag)" .)