Model clear top, showing muscle (8)
I believe the answer is:
pectoral
'muscle' is the definition.
(pectoral is a kind of muscle)
'model clear top' is the wordplay.
'model' is an anagram indicator.
'clear'+'top'='cleartop'
'cleartop' is an anagram of 'PECTORAL'.
'showing' acts as a link.
(Other definitions for pectoral that I've seen before include "Ornamental breastplate" , "Of the chest, say muscles" , "sort of cross" , "(Large muscle) of the chest" , "Alert cop about chest" .)