Size up edges of turned up Eton collar (4)
I believe the answer is:
rate
'size' is the definition.
(thesaurus)
'up edges of turned up eton collar' is the wordplay.
'up' becomes 'astir' (synonyms).
'edges of' suggests removing the centre.
'turned up' says the letters should be written backwards.
'eton collar' becomes 'te' (I can't justify this - if you can you should believe this answer much more).
'astir' with its middle taken out is 'ar'.
'ar' backwards is 'ra'.
'ra'+'te'='RATE'
Can you help me to learn more?
(Other definitions for rate that I've seen before include "Rank; speed" , "Appreciate" , "Give a value to something" , "The percentage of a loan charged as interest" , "Be worthy of; speed" .)