She's a vessel in the drink (5)
I believe the answer is:
tessa
'she's' is the definition.
'a vessel in the drink' is the wordplay.
'a vessel' becomes 'ss'.
'in the' means one lot of letters goes inside another.
'drink' becomes 'tea' (tea is a kind of drink**).
'ss' put inside 'tea' is 'TESSA'.
(Other definitions for tessa that I've seen before include "Girl's name, forerunner of ISA" , "Tax-exempt special savings account" , "Woman" , "Dahl or Jowell" , "Girl" .)