Diagram of vessel holding fifty (4)
I believe the answer is:
plan
'diagram' is the definition.
(I've seen this in another clue)
'vessel holding fifty' is the wordplay.
'vessel' becomes 'pan' (vessel or container for cooking).
'holding' means one lot of letters goes inside another.
'fifty' becomes 'l' (Roman numeral).
'pan' placed around 'l' is 'PLAN'.
'of' is the link.
(Other definitions for plan that I've seen before include "Design; map" , "Off the right direction" , "Large-scale map" , "Formulate" , "Devise a way to achieve" .)