Back in flat, select instrument (7)
I believe the answer is:
celesta
'instrument' is the definition.
(celesta is a kind of instrument)
'back in flat select' is the wordplay.
'back' says the letters should be written backwards.
'in' says the answer is hidden in the clue.
'atselec' is hidden within 'flat select'.
'atselec' reversed gives 'CELESTA'.
(Other definitions for celesta that I've seen before include "Keyboard percussion instrument giving a bell-like sound" , "Play this" , "Musical instrument" , "a bunch of keys?" , "one giving out notes" .)