Altos can take part in an opera (5)
I believe the answer is:
tosca
'an opera' is the definition.
(Giacomo Puccini opera)
'altos can take part' is the wordplay.
'take part' indicates the answer is hidden within the clue (I've seen 'taking part' mean this).
'TOSCA' is hidden within 'altos can'.
'in' acts as a link.
(Other definitions for tosca that I've seen before include "Puccini's jumper" , "musical work" , "Puccini's Roman heroine" , "Ascot (anag) - operatic heroine" , "Puccini's Rome opera" .)