Nearly a thousand missing outside (6)
I believe the answer is:
almost
'nearly' is the definition.
(synonyms)
'a thousand missing outside' is the wordplay.
'thousand' becomes 'm' (Roman numeral for a thousand).
'missing' becomes 'lost' (similar in meaning).
'outside' indicates putting letters inside (some letters go outside others).
'm' going inside 'lost' is 'lmost'.
'a'+'lmost'='ALMOST'
(Other definitions for almost that I've seen before include "As good as" , "Slightly short of" , "practically" , "Nearly, not quite" , "Not totally" .)