Cross old so-and-so retired with grin at last (5)
I believe the answer is:
tigon
'cross' is the definition.
(offspring of a lion and a tiger)
'old so-and-so retired with grin at last' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'so-and-so' becomes 'git' (I can't explain this - if you can you should believe this answer much more).
'retired' says the letters should be written in reverse.
'with' is a charade indicator (letters next to each other).
'at last' says to take the final letters.
The last letter of 'grin' is 'n'.
'o'+'git'='ogit'
'ogit' reversed gives 'tigo'.
'tigo'+'n'='TIGON'
Can you help me to learn more?
(Other definitions for tigon that I've seen before include "cat cross" , "Cross between tiger and lion" , "Hybrid offspring of two big cats" , "Tiger-lion hybrid" , "Animal" .)