Champion loses weight? It’s part of the target (5)
I believe the answer is:
inner
'part of the target' is the definition.
(ring of a target around the bullseye)
'champion loses weight?' is the wordplay.
'champion' becomes 'winner' (winner of a competition).
'loses' suggests deleting specific letters.
'weight?' becomes 'w' (short for weight).
'winner' with 'w' taken out is 'INNER'.
'it's' acts as a link.
(Other definitions for inner that I've seen before include "Central, internal" , "- - sanctum" , "Situated inside" , "Target next to bull" , "Farther in" .)