Sending back trifle once sick (10)
I believe the answer is:
reflection
'sending back' is the definition.
(reflect can mean to send back or reverse)
'trifle once sick' is the wordplay.
'sick' indicates an anagram.
'trifle'+'once'='trifleonce'
'trifleonce' with letters rearranged gives 'REFLECTION'.
(Other definitions for reflection that I've seen before include "Mirror image" , "mental study's required" , "serious thought?" , "Effect of not absorbing light" .)