Five sail out in vessels (5)
I believe the answer is:
vials
'vessels' is the definition.
(vial is a kind of vessel)
'five sail out in' is the wordplay.
'five' becomes 'v' (Roman numeral).
'out' indicates an anagram (out can mean wrong or inaccurate).
'in' is an insertion indicator.
'sail' with letters rearranged gives 'ials'.
'v' inserted into 'ials' is 'VIALS'.
(Other definitions for vials that I've seen before include "Small glass containers" , "Vessels" , "Ampoules" , "Small bottle that contains a drug" , "Small bottles of medicine" .)