Sour-tasting (4)
(Other definitions for tart that I've seen before include "Pie with fruit filling" , "Sharp; pastry" , "Sweet - sour" , "Acidulous" , "Sharp or acid tasting" .)
I believe this clue is a double definition.
'sour' is the first definition.
(I've seen this before)
'tasting' is the second definition.
(I have seen 'Sour tasting' mean 'acid' so perhaps 'tasting' could also mean 'acid')
(Other definitions for acid that I've seen before include "bad rain" , "Hallucinogenic drug" , "Sour (substance)" , "Sour tasting" , "Sour - type of rain or drop" .)