Our navy's active beyond Cape Horn (5)
I believe the answer is:
cornu
'horn' is the definition.
(I've seen this in another clue)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for cornu that I've seen before include "Hornblower may want it?" , "Outgrowth" , "In anatomy, any horn-shaped structure" .)