The one doing cartwheels, in blue (6)
I believe the answer is:
indigo
'blue' is the definition.
(I know that indigo is a type of blue)
'the one doing cartwheels' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
an anagram of 'doing' is 'ndigo' which is present in the answer.
No letters remain.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for indigo that I've seen before include "supplier of blue stuff" , "Deep violet-blue colour" , "Deep-blue dye or colour" , "Adulthood" , "Tropical plant" .)