He'll be only a second (6-2)
I believe the answer is:
runner-up
'a second' is the definition.
(I've seen this before)
'he'll be only' is the wordplay.
I cannot really see how this works, but
'be' could be 'run' (running is a kind of being) and 'run' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for runner-up that I've seen before include "Contestant finishing second" , "Second-best" , "Silver medal winner" , "Second-placed contestant" , "One nearly winning" .)