Group erected storage unit to support a citadel (9)
I believe the answer is:
acropolis
'citadel' is the definition.
(acropolis is a kind of citadel)
'group erected storage unit to support a' is the wordplay.
'group' becomes 'crop' (I can't explain this - if you can you should give a lot more credence to this answer).
'erected' says the letters should be written backwards.
'storage unit' becomes 'silo' (I've seen this before).
'to support' says to put letters next to each other.
'silo' written backwards gives 'olis'.
'crop'+'olis'='cropolis'
'cropolis' after 'a' is 'ACROPOLIS'.
Can you help me to learn more?
(Other definitions for acropolis that I've seen before include "Upper fortified part of Greek city - Athens's is famous" , "Citadel of ancient Greece" , "Athens landmark" , "Ancient citadel of Athens" , "high point" .)