Slow coach used single digit to round out trios (8)
I believe the answer is:
tortoise
'slow coach' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'single digit to round out trios' is the wordplay.
'single digit' becomes 'toe' (I've seen this in another clue).
'to round' is an insertion indicator.
'out' indicates an anagram.
'trios' is an anagram of 'rtois'.
'toe' enclosing 'rtois' is 'TORTOISE'.
'used' acts as a link.
Can you help me to learn more?
(Other definitions for tortoise that I've seen before include "Slow-moving land reptile" , "Shelled creature" , "Turtle-like reptile" , "Slow-moving four-footed reptile - outraced the hare" , "Slow reptile that raced a hare" .)