Alien found in Irish parliament? It's a fact! (6)
I believe the answer is:
detail
'it's a fact' is the definition.
(detail is a kind of fact)
'alien found in irish parliament?' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'found in' indicates putting letters inside.
'irish parliament?' becomes 'dail' (I've seen this before).
'et' inserted inside 'dail' is 'DETAIL'.
(Other definitions for detail that I've seen before include "analysts may go into" , "An individual item or element" , "Respect" , "Small individual feature" , "Dilate on particulars" .)