Calm gymnastics expert, nearly replete (8)
I believe the answer is:
peaceful
'calm' is the definition.
(similar in meaning)
'gymnastics expert nearly replete' is the wordplay.
'gymnastics' becomes 'pe' (Physical Education).
'expert' becomes 'ace' (ace can informally mean very good).
'nearly' means to remove the last letter.
'replete' becomes 'full' (synonyms).
'full' with its last letter taken away is 'ful'.
'pe'+'ace'+'ful'='PEACEFUL'
(Other definitions for peaceful that I've seen before include "Tranquil, calm" , "Not disturbed by strife or war" , "Placid, not belligerent" , "Quiet" , "Not violent" .)