Deceased not half needed for undertaking (4)
I believe the answer is:
deed
'undertaking' is the definition.
(thesaurus)
'deceased not half needed' is the wordplay.
'deceased' becomes 'dead' (similar in meaning).
'not half' means to take half the letters.
'needed' becomes 'ed' (I am not sure about this - if you are sure you should believe this answer much more).
'dead' with half the letters taken is 'de'.
'de'+'ed'='DEED'
'for' is the link.
Can you help me to learn more?
(Other definitions for deed that I've seen before include "Action - legal document" , "Document showing ownership" , "Act - exploit - legal document" , "Legal document for transfer of property" , "Something done" .)