Quality one's lacking in server (5)
I believe the answer is:
water
'quality' is the definition.
(eg the phrase 'finest water' meaning high quality)
'one's lacking in server' is the wordplay.
'one' becomes 'i' (roman numeral).
'lacking in' suggests deleting specific letters.
'server' becomes 'waiter' (synonyms).
'waiter' with 'i' taken away is 'WATER'.
(Other definitions for water that I've seen before include "Commonest fluid" , "Quality of diamonds" , "Aqua" , "Give fluid to (plants)" , "H2O" .)