Vessels held right in harbours (5)
I believe the answer is:
ports
'harbours' is the definition.
(I've seen this before)
'vessels held right in' is the wordplay.
'vessels' becomes 'pots' (pot is a kind of vessel).
'held right' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'pots' going around 'r' is 'PORTS'.
Can you help me to learn more?
(Other definitions for ports that I've seen before include "Harbours" , "Places where people and merchandise enter or leave a country" .)