"Don't ___" (3)
(Other definitions for ask that I've seen before include "Invite (to a party)" , "Something demanded" , "Make enquiry" , "Request, invite" , "Enquire" .)
'don't' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for taboo that I've seen before include "Inhibition based on social custom" , "(Something) forbidden" , "Not to be mentioned" , "Ritual prohibition" , "Socially forbidden" .)