Pointless fellow with posh hat (6)
I believe the answer is:
futile
'pointless' is the definition.
(thesaurus)
'fellow with posh hat' is the wordplay.
'fellow with' becomes 'f' (I've seen this in other clues).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'hat' becomes 'tile' (I've seen this before).
'f'+'u'+'tile'='FUTILE'
(Other definitions for futile that I've seen before include "Ineffective" , "Useless, a waste of effort" , "Trifling and ineffectual" , "Ineffectual, vain" , "If lute is altered it becomes ineffectual." .)