Rex gets stuck into finest French port (5)
I believe the answer is:
Here is my best explanation:
'french port' is the definition.
(a port city in brittany)
'rex gets stuck into finest' is the subsidiary indication.
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'gets stuck into' indicates putting letters inside.
'finest' becomes 'best' ('best' can be similar in meaning to 'finest').
'r' put inside 'best' is 'brest'.
(Other definitions for brest that I've seen before include "Breton port", "French Atlantic naval base", "Port in Brittany", "French warships here in Brittany", "Naval base in Brittany", "Brittany port" and "French port".)