Jumblies' vessel is rejected by first mate (5)
I believe the answer is:
sieve
'jumblies' vessel' is the definition.
(I've seen this before)
'is rejected by first mate' is the wordplay.
'rejected' shows that the letters should be reversed in order.
'by' says to put letters next to each other.
'first mate' becomes 'eve' (Adam's mate in the Bible).
'is' back-to-front is 'si'.
'si'+'eve'='SIEVE'
(Other definitions for sieve that I've seen before include "Forgetful head is like it" , "Jumblies' vessel" , "Craft for Jumblies" , "Device for separating lumps" , "Riddle, strain" .)