Lead-in to "X," "Y" or "Z" (3)
I believe the answer is:
gen
'lead-in to x y or z' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gen that I've seen before include "'Information, colloquially (3)'" , "Information (colloq.)" , "Lowdown (sl.)" , "Information yielded" , "Knowledge" .)