Cast racing round at first - that's natural (7)
I believe the answer is:
organic
'natural' is the definition.
(similar in meaning)
'cast racing round at first' is the wordplay.
'cast' indicates anagramming the letters (letters are dispersed or cast around).
'round' becomes 'o' (round can mean a round object).
'at first' means one lot of letters go next to another.
'racing' with letters rearranged gives 'rganic'.
'rganic' after 'o' is 'ORGANIC'.
'that's' acts as a link.
(Other definitions for organic that I've seen before include "No cigar (anag.)" , "Natural (farming)" , "with carbon involved" , "Vital - produced naturally" , "Of food, produced without pesticides or artificial fertilisers" .)