Developing naturally in cargo at sea (7)
I believe the answer is:
organic
'developing naturally' is the definition.
'in cargo at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'in'+'cargo'='incargo'
'incargo' is an anagram of 'ORGANIC'.
(Other definitions for organic that I've seen before include "without fertiliser" , "Produced by natural means" , "Vital - produced naturally" , "with carbon involved" , "(Of food) produced without synthetic fertilisers or pesticides" .)