Harpoons fish, filling ship (6)
I believe the answer is:
spikes
'harpoons' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Perhaps they are linked in a way I don't understand?
'fish filling ship' is the wordplay.
'fish' becomes 'pike' (pike is a kind of fish).
'filling' indicates putting letters inside.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'pike' inserted inside 'ss' is 'SPIKES'.
Can you help me to learn more?
(Other definitions for spikes that I've seen before include "Sharp points on athletes' shoes" , "Makes (a gun) useless" , "Running shoes" , "Prongs" .)