Fifty on board vessel providing storyline (4)
I believe the answer is:
plot
'storyline' is the definition.
(I've seen this before)
'fifty on board vessel' is the wordplay.
'fifty' becomes 'l' ('L' can be a synonym of 'fifty').
'on board' means one lot of letters goes inside another.
'vessel' becomes 'pot' (pot is a kind of vessel**).
'l' going inside 'pot' is 'PLOT'.
'providing' is the link.
(Other definitions for plot that I've seen before include "Ground - plan" , "intrigue" , "Make map" , "Small piece of ground" , "Secret scheme to do something underhand" .)