Incurable optimist experiencing brief pain over defeat (8) = PANGLOSS
brief pain = PANG, defeat = LOSS
Foolish constable, from NI district, skirting treacherous bog (8) = DOGBERRY
NI district = DERRY, treacherous = anagram (of big), A character from ‘Much Ado About Nothing’
Damage inside two Roman porcelain items? (5) = IMARI
damage = MAR, two Roman = II
PANGLOSS = incurable optimist cf PANGLOSSIAN
IMARI = porcelain items; type of Japanese porcelain.