Ill feeling of Heather keeping promise (8)
I believe the answer is:
loathing
'ill feeling' is the definition.
'heather keeping promise' is the wordplay.
'heather' becomes 'ling' (synonyms).
'keeping' means one lot of letters goes inside another.
'promise' becomes 'oath' (I've seen this before).
'ling' going around 'oath' is 'LOATHING'.
'of' is the link.
(Other definitions for loathing that I've seen before include "Detesting" , "Abhorrence" , "abomination" , "Hating" , "Abhorring" .)