Helper on vessel talked of for a number of years (6)
I believe the answer is:
decade
'number of years' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
(Other definitions for decade that I've seen before include "Tenth of a century" , "Group of ten things" , "The 1960s, for example" , "Ten consecutive years" , "number of years" .)