Sacrifice one during poor effort (7)
I believe the answer is:
forfeit
'sacrifice' is the definition.
(thesaurus)
'one during poor effort' is the wordplay.
'one' becomes 'i' (Roman numeral).
'during' is an insertion indicator.
'poor' is an anagram indicator.
'effort' is an anagram of 'forfet'.
'i' placed within 'forfet' is 'FORFEIT'.
(Other definitions for forfeit that I've seen before include "Something given up as a penalty" , "Give up or lose the right to something" , "It has to be paid or given up as a penalty" , "Surrender as a penalty" , "Effort I made to pay penalty" .)