Particle’s energy absorbed by quiet child (2,5)

'particle's' is the definition.
(mu-meson is a kind of particle)

'energy absorbed by quiet child' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'absorbed by' means one lot of letters goes inside another.
'quiet' becomes 'mum' (to keep mum is to stay quiet).
'child' becomes 'son' (male child).
'mum'+'son'='mumson'
'e' placed within 'mumson' is 'MU-MESON'.