Mostly remain around old Greek philosopher (5)
I believe the answer is:
stoic
'philosopher' is the definition.
(type of ancient philosopher)
'mostly remain around old greek' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'remain' becomes 'stick' (both can mean to stay).
'around' is an insertion indicator.
'old greek' becomes 'o' (I am not sure about this - if you are sure you should believe this answer much more).
'stick' with its final letter taken off is 'stic'.
'stic' placed around 'o' is 'STOIC'.
Can you help me to learn more?
(Other definitions for stoic that I've seen before include "Able to endure pain" , "Greek philosopher" , "Uncomplaining in suffering" , "Phlegmatic type" , "Resilient philosopher" .)