Elusive rogue dealer, half-hidden, nobody rumbled (12)
I believe the answer is:
imponderable
'elusive' is the definition.
(thesaurus)
'rogue dealer half-hidden nobody rumbled' is the wordplay.
'rogue' becomes 'imp'.
'half-hidden' means to take half the letters.
'rumbled' indicates an anagram (I've seen 'rumbling' mean this (I've seen this in other clues)).
'nobody' cut in half is 'nob'.
'dealer'+'nob'='dealernob'
'dealernob' with letters rearranged gives 'onderable'.
'imp'+'onderable'='IMPONDERABLE'
(Other definitions for imponderable that I've seen before include "Difficult to estimate" , "(Thing) that cannot be assessed" , "Difficult to assess" .)