Exit from garden behind shed I slam (9)
I believe the answer is:
castigate
'slam' is the definition.
(thesaurus)
'exit from garden behind shed i' is the wordplay.
I cannot quite understand how this works, but
'exit' could be 'gate' (I've seen this before) and 'gate' is present in the answer.
'shed' could be 'cast' (shed can mean to discard or cast off) and 'cast' is present in the answer.
'i' is within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for castigate that I've seen before include "Severely reprimand" , "Take to task" , "Berate" , "Punish" , "Take a piece out of" .)