Ready to tear bottom muscle? (4)
I believe the answer is:
ripe
'ready' is the definition.
(similar in meaning)
'tear bottom muscle?' is the wordplay.
'tear' becomes 'rip' (rip is a kind of tear).
'bottom' indicates one should take the final letters (I've seen 'bottom of' mean this).
The final letter of 'muscle' is 'e'.
'rip'+'e'='RIPE'
'to' acts as a link.
(Other definitions for ripe that I've seen before include "Fully developed or matured" , "Ready to pick" , "Mature, ready to eat" , "Mature for eating like fruit" , "daring" .)