They help to run vessel laden with fish (6)
I believe the answer is:
spikes
'they help to run' is the definition.
(as in spikes on running shoes)
'vessel laden with fish' is the wordplay.
'vessel' becomes 'SS' (prefix in ship names eg SS Great Britain).
'laden with' indicates putting letters inside (some letters are loaded up with others).
'fish' becomes 'pike' (pike is a kind of fish).
'ss' going around 'pike' is 'SPIKES'.
(Other definitions for spikes that I've seen before include "Makes (a gun) useless" , "Sharp points on athletes' shoes" , "Running shoes" , "Prongs" .)