Mo's meaning — at first — men doubly misinterpreted (10)
I believe the answer is:
molybdenum
'mo's' is the definition.
(synonyms)
'meaning at first men doubly misinterpreted' is the wordplay.
'at first' indicates taking the first letters.
'misinterpreted' is an anagram indicator.
The initial letter of 'meaning' is 'm'.
'men'+'doubly'='mendoubly'
'mendoubly' is an anagram of 'olybdenum'.
'm'+'olybdenum'='MOLYBDENUM'
(Other definitions for molybdenum that I've seen before include "metal? Quite!" , "Metallic element" .)