Find Matilda's a police officer (9)
I believe the answer is:
detective
'a police officer' is the definition.
(I know that detective is a type of officer)
'find matilda's' is the wordplay.
'find' becomes 'detect' (synonyms).
'matilda's' becomes 'I've' (where the crossword setter is 'Matilda').
'detect'+'ive'='DETECTIVE'
(Other definitions for detective that I've seen before include "Policeman, say" , "Public or private investigator" , "Expert solver of clues" , "Private or police investigator" , "Investigating policeman" .)