When to get son donkey (3)
I believe the answer is:
ass
'donkey' is the definition.
(ass is a term for a donkey)
'when to get son' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'to get' is a charade indicator (letters next to each other).
'son' becomes 's' (genealogical abbreviation for son).
'as'+'s'='ASS'
(Other definitions for ass that I've seen before include "Donkey - one that's foolish" , "Donkey or foolish person" , "Stupid person or American bum" , "Stupid fellow" , "Democratic mascot" .)