Port of Spain coastguard's first one on board US cutter (5)
I believe the answer is:
cadiz
'port of spain' is the definition.
(I've seen this in another clue)
'coastguard's first one on board us cutter' is the wordplay.
'coastguard's first' becomes 'c' (ast letter of 'coastguard').
'one' becomes 'i' (Roman numeral).
'on board' indicates putting letters inside (inserted letters are taken on board).
'us cutter' becomes 'adz' (I can't explain this - if you can you should give a lot more credence to this answer).
'i' placed within 'adz' is 'adiz'.
'c'+'adiz'='CADIZ'
Can you help me to learn more?
(Other definitions for cadiz that I've seen before include "European port" , "Old Spanish port" , "Andalusian port" , "Old port of south-west Spain" , "Southwest Spanish port" .)