Pointless being deceitful about forgetting last month (6)
I believe the answer is:
futile
'pointless' is the definition.
(thesaurus)
'last month' is the wordplay.
'last' becomes 'f' (I've seen this in other clues).
'month' becomes 'utile' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'f'+'utile'='FUTILE'
'being deceitful about forgetting' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Other definitions for futile that I've seen before include "Ineffectual, vain" , "In vain, a waste of time" , "Useless or vain" , "If lute is altered it becomes ineffectual." , "Ineffectual,in vain" .)