Cancel woman, extremely unhelpful (5)
I believe the answer is:
annul
'cancel' is the definition.
(I know that annul is a more specific form of cancel)
'woman extremely unhelpful' is the wordplay.
'woman' becomes 'ann' (woman's name).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'unhelpful' with its middle removed is 'ul'.
'ann'+'ul'='ANNUL'
(Other definitions for annul that I've seen before include "Scrap" , "Recall" , "Declare void" , "Cancel, abolish" , "Cancel, say a law" .)