Blue-green coats in fashion worn by Navy specialist (9)
I believe the answer is:
technical
'specialist' is the definition.
(I've seen this before)
'blue-green coats in fashion worn by navy' is the wordplay.
'blue green' becomes 'teal' (I've seen this in another clue).
'coats' means one lot of letters goes inside another (inserted letters are coated in another word).
'in fashion' becomes 'chic' (I've seen this in another clue).
'worn by' is an insertion indicator (outside letters are worn by those on the inside).
'navy' becomes 'N' (common abbreviation eg in RN Royal Navy).
'chic' going around 'n' is 'chnic'.
'teal' going around 'chnic' is 'TECHNICAL'.
(Other definitions for technical that I've seen before include "specialising in applied science" , "Relating to mechanical art or applied science" , "requiring special skill" , "Industrial" , "Concerned with applied science" .)