Beat poets essentially in a state of agitation (7)
I believe the answer is:
leather
'beat' is the definition.
(to leather can mean to beat or thrash)
'poets essentially in a state of agitation' is the wordplay.
'essentially' indicates the central letters.
'in' indicates putting letters inside.
'a state of agitation' becomes 'lather' (as in the phrase 'in a lather').
The middle of 'poets' is 'e'.
'e' placed within 'lather' is 'LEATHER'.
(Other definitions for leather that I've seen before include "durable material" , "kid, perhaps" , "calf" , "Fabric" , "Animal skins" .)