Essential element of returned contract summarised in part (4)
I believe the answer is:
must
'essential element' is the definition.
(a must is something essential)
'returned contract summarised in part' is the wordplay.
'returned' says the letters should be written backwards.
'in part' indicates the answer is hidden within the clue.
'tsum' is hidden within 'contract summarised'.
'tsum' in reverse letter order is 'MUST'.
'of' acts as a link.
(Other definitions for must that I've seen before include "Is obliged to" , "The imperative for mould" , "Grape juice ready for fermentation" , "Essential to have" , "Necessity" .)