Flag officer supports right one entering service abroad? (9)
I believe the answer is:
tricolour
'flag' is the definition.
(tricolour is a kind of flag)
'officer supports right one entering service abroad?' is the wordplay.
'officer' becomes 'col' (I've seen this before).
'supports' says to put letters next to each other (bottom letters provide a support for the top ones).
'right one' becomes 'ri' (I can't justify this - if you can you should give a lot more credence to this answer).
'entering' means one lot of letters goes inside another.
'service abroad?' becomes 'tour' (type of service which might be obtained abroad).
'col' after 'ri' is 'ricol'.
'ricol' inserted within 'tour' is 'TRICOLOUR'.
(Other definitions for tricolour that I've seen before include "Standard of French perhaps" , "Having or involving three colours" , "French national flag" .)