Lilac rim on yam's inside reveals bitterness (8)
I believe the answer is:
acrimony
'bitterness' is the definition.
('acrimony' can be a synonym of 'bitterness')
'lilac rim on yam's inside' is the wordplay.
'inside' says the answer is hidden in the clue.
'ACRIMONY' can be found hidden inside 'lilac rim on yams'.
'reveals' is the link.
(Other definitions for acrimony that I've seen before include "I cry, moan (anag)" , "Acronym I coined in hostility" , "Bad feeling" , "Acronym I construct with bitter words" , "'Bitterness, sharpness (8)'" .)