Yes, an apple core is not greatly refreshing (3)
I believe the answer is:
sip
'not greatly refreshing' is the definition.
I can't tell whether this defines the answer.
'yes an apple core' is the wordplay.
'yes' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'an' becomes 'i' (Roman numeral for one).
'core' means to look at the middle letters.
The middle of 'apple' is 'p'.
's'+'i'+'p'='SIP'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for sip that I've seen before include "Drink slowly" , "Take a little of a drink" , "Take small mouthfuls of (drink)" , "Tiny amount of drink" , "Thimbleful" .)