Instrument back in that selection (7)
I believe the answer is:
celesta
'instrument' is the definition.
(I know that celesta is a type of keyboard instrument)
'back in that selection' is the wordplay.
'back' says the letters should be written backwards.
'in' indicates the answer is hidden within the clue.
'atselec' can be found hidden inside 'that selection'.
'atselec' backwards is 'CELESTA'.
(Other definitions for celesta that I've seen before include "a bunch of keys?" , "Keyboard percussion instrument giving a bell-like sound" , "Musical instrument" , "Play this" , "one giving out notes" .)