My lost puzzle, mainly (6)
I believe the answer is:
mostly
'mainly' is the definition.
(thesaurus)
'my lost puzzle' is the wordplay.
'puzzle' is an anagram indicator.
'my'+'lost'='mylost'
'mylost' is an anagram of 'MOSTLY'.
(Other definitions for mostly that I've seen before include "In large part" , "On the whole" , "In the main, nearly all" , "Largely" , "More often than not" .)