Trial involved old part of Venice (6)
I believe the answer is:
rialto
'part of venice' is the definition.
'trial involved old' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'old' becomes 'o' (abbreviation).
'trial' anagrammed gives 'rialt'.
'rialt'+'o'='RIALTO'
(Other definitions for rialto that I've seen before include "Venetian island" , "famous city district" , "Region in Venice and Dublin" , "Area famous for 7 [ABRIDGE]" , "Dublin district, or Shakespeare's 'What news on the . . . . . .?'" .)