Mistakenly marries best man losing one's shame (13)
I believe the answer is:
embarrassment
'shame' is the definition.
(embarrassment is a kind of shame)
'mistakenly marries best man losing one's' is the wordplay.
'mistakenly' is an anagram indicator.
'losing' is a deletion indicator.
'one' becomes 'i' (Roman numeral).
'marries'+'best'+'man'='marriesbestman'
'marriesbestman' with 'i' taken away is 'marresbestman'.
'marresbestman' is an anagram of 'EMBARRASSMENT'.
(Other definitions for embarrassment that I've seen before include "Discomposure" , "Financial difficulty; feeling of shame" , "Surplus" , "Awkwardness, discomfiture" , "Self-consciousness, shame" .)