Dock a navy's vessels (9)
I believe the answer is:
trimarans
'vessels' is the definition.
(trimaran is a kind of vessel)
'dock a navy's' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for trimarans that I've seen before include "Sailing boats with 3 hulls" , "Sailboats with three parallel hulls" .)