Almost lost gymnast’s first medal (4)
I believe the answer is:
gong
'medal' is the definition.
(I've seen this before)
'almost lost gymnast's first' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'lost' becomes 'gone' ('gone' can be a synonym of 'lost').
'first' says to take the initial letters.
The first letter of 'gymnasts' is 'g'.
'gone' with its final letter taken away is 'gon'.
'gon'+'g'='GONG'
(Other definitions for gong that I've seen before include "Percussion instrument - medal (slang)" , "Sound it, for dinner say" , "Resonant metal disc" , "Dinner instrument?" , "Medal; forty down" .)