Model Jean, losing weight, is a diminutive sort (6)
I believe the answer is:
shrimp
'diminutive sort' is the definition.
(shrimp matches up with the definition diminutive sort)
'model jean losing weight' is the wordplay.
'model jean' becomes 'shrimpton' (Jean Shrimpton, model of the 1960s).
'losing' is a deletion indicator.
'weight' becomes 'ton' (unit of weight).
'shrimpton' with 'ton' taken away is 'SHRIMP'.
'is a' is the link.
(Other definitions for shrimp that I've seen before include "Tasty crustacean found in cocktails" , "Diminutive person" , "colour?" , "Small, weak person" , "tiny chap" .)