Turn on alien instrument (6)
I believe the answer is:
spinet
'instrument' is the definition.
(spinet is a kind of instrument)
'turn on alien' is the wordplay.
'turn' becomes 'spin' (spin is a kind of turn**).
'on' is a charade indicator (letters next to each other).
'alien' becomes 'et' (extra terrestrial).
'spin'+'et'='SPINET'
(Other definitions for spinet that I've seen before include "Early keyboard instrument" , "Small 12" , "Step in with old harpsichord" , "Harpsichord-like keyboard instrument" , "Step in with old musical instrument" .)