The only one in a suit (3)
I believe the answer is:
ace
'one in a suit' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ace that I've seen before include "Top playing card" , "strained" , "Single unit on playing cards" , "single > club > champion" , "first-class" .)