Originally, offers by upwardly mobile online marketplace confined to hospital food (4,4)
I believe the answer is:
soya bean
'originally offers by' is the definition.
I can't judge whether this definition defines the answer.
'upwardly mobile online marketplace confined to hospital food' is the wordplay.
'upwardly mobile' says the letters should be written backwards.
'online marketplace' becomes 'ebay' (I've seen this in another clue).
'confined to' is an insertion indicator.
'hospital food' becomes 'naos' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ebay' inserted into 'naos' is 'naebayos'.
'naebayos' written backwards gives 'SOYA-BEAN'.
Can you help me to learn more?
(Other definitions for soya bean that I've seen before include "valuable source of protein" , "Legume" , "Protein-rich seed used in making flour, milk etc." , "Protein-rich veg." , "Plant protein source" .)