Withstand having been separated - it's beastly (6)
I believe the answer is:
weasel
'withstand having' is the definition.
I can't judge whether this defines the answer.
'been separated it's beastly' is the wordplay.
'been' becomes 'was' (being is a kind of being).
'separated' is an insertion indicator.
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'beastly' becomes 'el' (I can't justify this - if you can you should believe this answer much more).
'was' going around 'e' is 'weas'.
'weas'+'el'='WEASEL'
Can you help me to learn more?
(Other definitions for weasel that I've seen before include "Sly type" , "Rat" , "Stoat-like animal" , "Musteline animal" , "Proverbially deceitful mammal" .)