Mr Collins has a much less vibrant description in his excellent book of words
the substitution of a word referring to an attribute for the thing that is meant, as for example the use of the crown to refer to a monarchHowever, I was instructed at the end of this definition to - compare synecdoche!
You can not go around giving orders like that to a young lad without there being consequences. I confess I have wrestled with synecdoche almost as long as I have wrestled with the demon drink and sins of the flesh. I have used mortification of the body and spiritual exercises to cope with this word but it is no use. It has me defeated.
Mr Collins may briefly define it thus
a figure of speech in which a part is substituted for a whole or a whole for a part, as in 50 head of cattle for 50 cows, or the army for a soldierand be back at the rashers and eggs in no time, washing it down with lashings of tea I’m sure, but I am destroyed. The little worm is there; the tautological quantum elusiveness of the smile on Schrödinger’s Cat:- a part is substituted for a whole or a whole for a part
I’m extirpated!
(Ed. There, there. Come and sit down and have a cup of tea. Do you want me to call your Mammy?)