Pen Charlie essentially twirled in exam (6)
I believe the answer is:
corral
'pen' is the definition.
(both are a type of enclosure for animals)
'charlie essentially twirled in exam' is the wordplay.
'charlie' becomes 'C' (phonetic alphabet: alpha, bravo, charlie etc.).
'essentially' indicates the central letters.
'in' means one lot of letters goes inside another.
'exam' becomes 'oral' (spoken academic test).
The centre of 'twirled' is 'r'.
'r' put within 'oral' is 'orral'.
'c'+'orral'='CORRAL'
(Other definitions for corral that I've seen before include "Round up" , "Pen for cattle - ring of waggons" , "Pen for livestock (US)" , "Horse enclosure out West" , "Pen for cattle in the US" .)