Crop unit planted in garden complex (5)
I believe the answer is:
maize
'crop' is the definition.
(I've seen this before)
'unit planted in garden complex' is the wordplay.
'unit' becomes 'I' (synonyms).
'planted in' indicates putting letters inside.
'garden complex' becomes 'maze' (I can't justify this - if you can you should give a lot more credence to this answer).
'i' placed into 'maze' is 'MAIZE'.
Can you help me to learn more?
(Other definitions for maize that I've seen before include "Indian corn" , "Cereal crop" , "Corn-cob cereal" , "plant with ears" , "Tall cereal grass producing large edible ears" .)