Muscle vacuous: dangerous second prisoner held, escapes (8)
I believe the answer is:
absconds
'escapes' is the definition.
'muscle vacuous dangerous second prisoner held' is the wordplay.
'muscle' becomes 'ab' (short for abdominal muscle).
'vacuous' says to hollow out the word (remove centre letters).
'second' becomes 's' (common abbreviation).
'prisoner' becomes 'con' (abbreviation for 'convict').
'held' is an insertion indicator.
'dangerous' with its middle taken out is 'ds'.
's'+'con'='scon'
'ab'+'ds'='abds'
'abds' enclosing 'scon' is 'ABSCONDS'.
(Other definitions for absconds that I've seen before include "Flees to escape from a legal process" , "Bolts, runs away" , "Does a bunk" , "Decamps" , "Flees, runs away" .)