Muscle spasm’s beginning in retired singer? (8)

'muscle' is the definition.
(extensor muscle is a kind of muscle)

'spasm's beginning in retired singer?' is the wordplay.
'beginning' suggests taking the first letters.
'in' is an insertion indicator.
'retired' becomes 'ex' (prefix meaning former or onetime).
'singer?' becomes 'tenor' (tenor is a kind of singer).
The initial letter of 'spasms' is 's'.
'ex'+'tenor'='extenor'
's' put inside 'extenor' is 'EXTENSOR'.