Figures sit back during study of bodies at rest (10)
I believe the answer is:
statistics
'figures' is the definition.
(statistical figures)
'sit back during study of bodies at rest' is the wordplay.
'back' says the letters should be written in reverse.
'during' is an insertion indicator.
'study of bodies at rest' becomes 'statics' (scientific study of static bodies).
'sit' in reverse letter order is 'tis'.
'tis' put inside 'statics' is 'STATISTICS'.
(Other definitions for statistics that I've seen before include "'20, 16 20, and _____'" , "(Study of) quantitative data" , "results" , "Branch of maths" .)