Citadel in a harvest on poor soil (9)
I believe the answer is:
acropolis
'citadel' is the definition.
(acropolis is a kind of citadel)
'a harvest on poor soil' is the wordplay.
'harvest' becomes 'crop' (synonyms).
'on' indicates putting letters inside.
'poor' is an anagram indicator.
'soil' anagrammed gives 'olis'.
'crop' placed into 'olis' is 'cropolis'.
'a'+'cropolis'='ACROPOLIS'
'in' is the link.
(Other definitions for acropolis that I've seen before include "place of high security" , "Upper fortified part of Greek city - Athens's is famous" , "Upper fortified part of Greek city as in Athens" , "old citadel" , "Parthenon location" .)