Donkey jacket belonging to aimless son (3)
I believe the answer is:
ass
'donkey' is the definition.
(ass is a term for a donkey)
'jacket belonging to aimless son' is the wordplay.
'jacket belonging to' suggests removing the centre (the outside or jacket of the word is used).
'son' becomes 's' (genealogical abbreviation for son).
'aimless' with its centre taken out is 'as'.
'as'+'s'='ASS'
(Other definitions for ass that I've seen before include "Silly fellow in passionate state" , "Horse hybrid" , "Donkey or foolish person" , "Blockhead" , "Donkey - one that's foolish" .)