Unfortunately lost a great deal (4)
I believe the answer is:
lots
'a great deal' is the definition.
(I've seen this in another clue)
'unfortunately lost' is the wordplay.
'unfortunately' indicates an anagram (a bad or unfortunate spelling).
'lost' is an anagram of 'LOTS'.
(Other definitions for lots that I've seen before include "A great deal" , "Oodles" , "a crowd?" , "may be drawn" , "Loads" .)