Son has left aforementioned charity (3)
I believe the answer is:
aid
'charity' is the definition.
(I've seen this before)
'son has left aforementioned' is the wordplay.
'son' becomes 's'.
'has left' suggests deleting specific letters.
'aforementioned' becomes 'said' (synonyms).
'said' with 's' removed is 'AID'.
(Other definitions for aid that I've seen before include "grant" , "Auxiliary" , "Help or succour" , "Relief" , "charity" .)