I'm blocking a friend's ill-feeling (8)
I believe the answer is:
acrimony
'ill-feeling' is the definition.
(I've seen this before)
'i'm blocking a friend's' is the wordplay.
'blocking' means one lot of letters goes inside another.
'friend' becomes 'crony' (crony is a kind of friend).
'a'+'crony'='acrony'
'im' going inside 'acrony' is 'ACRIMONY'.
(Other definitions for acrimony that I've seen before include "Bitterness of temper" , "Bitterness of manner or words" , "Angry and unpleasant feelings or words" , "I cry, moan (anag)" , "poison" .)