Strange epic in books ends with muscle (6)
I believe the answer is:
biceps
'muscle' is the definition.
(biceps is a kind of muscle)
'strange epic in books ends' is the wordplay.
'strange' indicates anagramming the letters.
'in' is an insertion indicator.
'ends' means to remove the middle letters.
'books' with its centre removed is 'bs'.
'epic' with letters rearranged gives 'icep'.
'icep' placed into 'bs' is 'BICEPS'.
'with' is the link.
(Other definitions for biceps that I've seen before include "One operating arm" , "Contractor" , "Weightlifters use them" , "Arm workers" , "Arm muscles" .)