Body kept inside bank (4)
I believe the answer is:
dyke
'bank' is the definition.
'body kept inside' is the wordplay.
'inside' indicates a hidden word.
'DYKE' is hidden within 'body kept'.
(Other definitions for dyke that I've seen before include "Anti-flood ditch/wall" , "Flood embankment" , "defensive wall" , "Offa's barrier?" , "Embankment built to prevent flooding as in Holland" .)