Lost out a great deal (4)
I believe the answer is:
lots
'a great deal' is the definition.
(I've seen this before)
'lost out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'lost' anagrammed gives 'LOTS'.
(Other definitions for lots that I've seen before include "Large numbers at auction" , "Great quantities" , "a crowd?" , "Something to draw" , "Oodles" .)