Good old Nell only half dead (4)
I believe the answer is:
gone
'dead' is the definition.
(similar in meaning)
'good old nell only half' is the wordplay.
'good' becomes 'g' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'only half' means to take half the letters.
'nell' with half the letters taken is 'ne'.
'g'+'o'+'ne'='GONE'
(Other definitions for gone that I've seen before include "Departed or no longer available" , "... saga of 2's civil war" , "Departed or dead" , "Away, departed" , "Departed; used up" .)