Tricky problem of stolen vessel at bottom of Sargasso (3,6)
I believe the answer is:
hot potato
'tricky problem' is the definition.
(I've seen this before)
'stolen vessel at bottom of sargasso' is the wordplay.
'stolen' becomes 'hot' (I've seen this before).
'vessel' becomes 'pot' (pot is a kind of vessel**).
'bottom of' indicates one should take the final letters.
The last letter of 'sargasso' is 'o'.
'hot'+'pot'+'at'+'o'='HOT POTATO'
'of' acts as a link.
(Other definitions for hot potato that I've seen before include "contentious question" , "Problem that's painful to grasp" , "Controversial issue (colloq.)" , "controversy" , "A difficult situation" .)