Odd shape or top mark (10)
I believe the answer is:
apostrophe
'mark' is the definition.
(punctuation mark)
'odd shape or top' is the wordplay.
'odd' indicates anagramming the letters.
'shape'+'or'+'top'='shapeortop'
'shapeortop' anagrammed gives 'APOSTROPHE'.
(Other definitions for apostrophe that I've seen before include "Part of man's" , "Often misplaced punctuation mark" , "It may be superfluous on tradesman's sign" , "Mark in exam's" , "Where character may be absent" .)