Select a complicated instrument (7)
I believe the answer is:
celesta
'instrument' is the definition.
(celesta is a kind of instrument)
'select a complicated' is the wordplay.
'complicated' is an anagram indicator.
'select'+'a'='selecta'
'selecta' anagrammed gives 'CELESTA'.
(Other definitions for celesta that I've seen before include "Musical instrument" , "Keyboard percussion instrument giving a bell-like sound" , "one giving out notes" , "a bunch of keys?" , "Play this" .)