As a Kent prison closing early, run to get inside (7)
I believe the answer is:
arsenic
'as' is the definition.
(As is the chemical symbol for arsenic)
'a kent prison closing early run to get inside' is the wordplay.
'a kent' becomes 'ase' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'prison' becomes 'nick' (slang term for prison).
'closing early' means to remove the last letter.
'run' becomes 'r' (cricket abbreviation).
'to get inside' is an insertion indicator.
'nick' with its last letter taken away is 'nic'.
'ase'+'nic'='asenic'
'asenic' placed around 'r' is 'ARSENIC'.
Can you help me to learn more?
(Other definitions for arsenic that I've seen before include "Poison used in pesticides and weedkillers" , "Element with many poisonous compounds" , "killer" , "Ratsbane" , "Toxic metalloid element" .)