Most pleasant characters among Venice's tourists (6)
I believe the answer is:
nicest
'most pleasant' is the definition.
('nice' can be similar in meaning to 'pleasant')
'characters among venice's tourists' is the wordplay.
'characters' indicates a hidden word (some letters or characters from the clue).
'NICEST' can be found hidden inside 'among venices tourists'.
(Other definitions for nicest that I've seen before include "Best" , "Sin etc (anag)" , "Most pleasing" , "Most pleasant or fine" , "Most appealing" .)