1000 is suitable for a square peg (6)
I believe the answer is:
misfit
'a square peg' is the definition.
(I've seen this in another clue)
'1000 is suitable' is the wordplay.
'1000' becomes 'M' (roman numeral).
'suitable' becomes 'fit' (similar in meaning).
'm'+'is'+'fit'='MISFIT'
'for' acts as a link.
(Other definitions for misfit that I've seen before include "Social outcast" , "Weirdo" , "One can't adapt to their circumstances" , "One who is out of place" , "Maverick, nonconformist" .)