At the back, small glass instruments (6)
I believe the answer is:
flutes
'instruments' is the definition.
(flute is a kind of instrument)
'at the back small glass' is the wordplay.
'at' is a charade indicator (letters next to each other).
'the back small' becomes 's' (abbreviation - e.g. clothes size. I am not sure about the 'back' bit.).
'glass' becomes 'flute glass' (flute glass is a kind of glass).
's' after 'flute' is 'FLUTES'.
Can you help me to learn more?
(Other definitions for flutes that I've seen before include "High-pitched woodwind instruments" , "Wind instruments as played by James Galway" , "Pipe-like musical instruments" , "Musical instruments as for James Galway" .)