Defeat current racket appearing in The Mercury (6)
I believe the answer is:
hiding
'defeat' is the definition.
(I have seen 'Severe defeat ' mean 'hiding' so perhaps 'defeat' could also mean 'hiding')
'current racket appearing in the mercury' is the wordplay.
'current' becomes 'i' (symbol for electric current).
'racket' becomes 'din' (I've seen this before).
'appearing in' indicates putting letters inside.
'the mercury' becomes 'hg' (Hg is the chemical symbol for mercury).
'i'+'din'='idin'
'idin' going within 'hg' is 'HIDING'.
(Other definitions for hiding that I've seen before include "Beating; severe defeat" , "physical punishment" , "Harm, damage" , "Secreting" , "corporal punishment" .)