Undermine writer's mate (6)
I believe the answer is:
impair
'undermine' is the definition.
(I've seen this in another clue)
'writer's mate' is the wordplay.
'writer's' becomes 'I'm' ('I am' referring to the writer of the clue).
'mate' becomes 'pair' (synonyms).
'im'+'pair'='IMPAIR'
(Other definitions for impair that I've seen before include "Cause weakening of, damage" , "Spoil, degrade" , "Hobble" , "Make worse or less effective" , "Damage or injure" .)