Capture first couple turning back - that's most unusual (6)
I believe the answer is:
rarest
'that's most unusual' is the definition.
(rare can mean unusual or odd)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for rarest that I've seen before include "Least common, most unusual" , "Most scarce or least common" , "Most scarce and probably precious" , "Most scarce or uncommon" , "least done" .)