Former lover's not embellished son's excuses (8)
I believe the answer is:
explains
'excuses' is the definition.
(I know this)
'former lover's not embellished son's' is the wordplay.
'former lover' becomes 'ex' (an ex-lover).
'not embellished' becomes 'plain' (not embellished or decorated).
'ex'+'plain'+'s'='EXPLAINS'
(Other definitions for explains that I've seen before include "Gives reasons for" , "Shows the meaning of, or accounts for" , "Makes comprehensible" , "Makes intelligible" , "Makes something clear" .)