Sent round specimens essentially taken as given (7)
I believe the answer is:
posited
'taken as given' is the definition.
(positing is a kind of taking as given)
'sent round specimens essentially' is the wordplay.
'sent' becomes 'posted' ('post' can be a synonym of 'send').
'round' means one lot of letters goes inside another.
'specimens essentially' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'posted' placed around 'i' is 'POSITED'.
Can you help me to learn more?
(Other definitions for posited that I've seen before include "Advanced" , "Assumed" , "Topside (anag.)" , "Proposed as basis for argument" , "Put forward as fact or as a basis for argument" .)