Sweet vessel that's capsized must be made stronger (10)
I believe the answer is:
buttressed
'made stronger' is the definition.
(to buttress something is to make it stronger)
'sweet vessel that's capsized' is the wordplay.
'sweet' becomes 'dessert' (synonyms).
'vessel' becomes 'tub' (tub is a kind of vessel).
'that's capsized' says the letters should be written backwards (in a down clue, the word turns upside down).
'dessert'+'tub'='desserttub'
'desserttub' in reverse letter order is 'BUTTRESSED'.
'must be' acts as a link.
(Other definitions for buttressed that I've seen before include "held securely" , "Supported" , "Reinforced" .)