Muscles stretch, in summary (8)
I believe the answer is:
abstract
'summary' is the definition.
(I know that abstract is a type of summary)
'muscles stretch' is the wordplay.
'muscles' becomes 'abs' (short for abdominals).
'stretch' becomes 'tract' (a tract is a stretch of land).
'abs'+'tract'='ABSTRACT'
'in' is the link.
(Other definitions for abstract that I've seen before include "Take away - immaterial" , "Theoretical, not concrete" , "hard to understand" , "Appropriate - theoretical - summary" , "Concerned with theories rather than things" .)