Tart and custard isn’t desirable in courses — just starters (6)
I believe the answer is:
acidic
'tart' is the definition.
(I've seen this before)
'and custard isn't desirable in courses just starters' is the wordplay.
'just starters' suggests taking the first letters (only the letters which start the words).
The initial letters of 'and custard isnt desirable in courses' is 'ACIDIC'.
(Other definitions for acidic that I've seen before include "Unpleasant" , "Tasting sharp or sour" , "Containing a sour-tasting substance" , "Like vinegar" , "Sharp-tasting, sour" .)