Nationalist party overwhelmed by Irish support (7)
I believe the answer is:
'support' is the definition.
(endorsing is a kind of supporting)
'nationalist party overwhelmed by irish' is the wordplay.
'nationalist' becomes 'n' (abbreviation in politics).
'party' becomes 'do' (do is a kind of party).
'overwhelmed by' means one lot of letters goes inside another (I've seen this in other clues).
'irish' becomes 'erse' (I've seen this before).
'ndo' inserted within 'erse' is 'ENDORSE'.
(Other definitions for endorse that I've seen before include "Give backing to" , "'Approve of, back (7)'" , "Approve or underwrite" , "No reeds (anag)" , "Sanction -- red nose (anag)" .)