Son dispatching one third of Brandy Alexander (5)
I believe the answer is:
sandy
'alexander' is the definition.
(nickname for Alexander)
'son dispatching one third of brandy' is the wordplay.
I cannot really see how this works, but
'son' could be 's' (genealogical abbreviation for son) and 's' is found within the answer.
'one' could be 'an' (an apple is one apple) and 'an' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sandy that I've seen before include "- - Lyle (golf)" , "Like a beach perhaps" , "Light yellowish brown" , "Boy's or girl's name -- like a beach" , "Hair colour" .)