Was left in by the lady with the diet problem (9)
I believe the answer is:
inherited
'was left' is the definition.
(I've seen this before)
'in by the lady with the diet problem' is the wordplay.
'by' means one lot of letters go next to another.
'the lady' becomes 'her' (I've seen this before**).
'with' is a charade indicator (letters next to each other).
'problem' is an anagram indicator.
'diet' is an anagram of 'ited'.
'in'+'her'+'ited'='INHERITED'
(Other definitions for inherited that I've seen before include "Came into" , "Received from an ancestor" , "Was left" , "handed down" , "Received from forebears" .)