Nationalist party overwhelmed by Irish support (7)
I believe the answer is:
endorse
'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).
'n'+'do'='ndo'
'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)" .)