Being in a race, their costs are exceptional (5)
I believe the answer is:
scots
'being in a race' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'costs are exceptional' is the wordplay.
'are exceptional' is an anagram indicator (an unusual or exceptional ordering of the letters).
'costs' with letters rearranged gives 'SCOTS'.
'their' acts as a link.
Can you help me to learn more?
(Other definitions for scots that I've seen before include "Taxes" , "A UK language" , "- Greys, - pine" , "People from north of border" , "North British" .)