Watery grave doesn't have one (6)
I believe the answer is:
serous
'watery' is the definition.
(thesaurus)
'grave doesn't have one' is the wordplay.
'grave' becomes 'serious' ('serious' can be similar in meaning to 'grave').
'doesn't have' indicates named letters should be taken away.
'one' becomes 'i' (Roman numeral).
'serious' with 'i' removed is 'SEROUS'.
(Other definitions for serous that I've seen before include "Relating to the liquid in blood" , "Watery" .)