Access inside of sent box, perhaps (6)
I believe the answer is:
entree
'access' is the definition.
(I know that entree is a type of access)
'inside of sent box perhaps' is the wordplay.
'inside of' says to take the centre.
'box perhaps' becomes 'tree' (box tree is a type of tree).
The middle letters of 'sent' are 'en'.
'en'+'tree'='ENTREE'
(Other definitions for entree that I've seen before include "Dish allowing one to go in?" , "The right ot privilege of admission" , "Introduction" , "Main course dish" , "Dish served between fish and meat course" .)