Irish American crushed by very nasty bug (5)
I believe the answer is:
virus
'nasty bug' is the definition.
(bug can mean an infection)
'irish american crushed by very' is the wordplay.
'irish' becomes 'IR' (abbreviation).
'american' becomes 'US' (abbreviation for United States).
'crushed by' says to put letters next to each other (in a down clue, some letters go on top of others).
'very' becomes 'v' (abbreviation).
'ir'+'us'='irus'
'irus' put after 'v' is 'VIRUS'.
(Other definitions for virus that I've seen before include "One infecting human, computer" , "Transmitted cause of infection" , "Microscopic infectant" , "horror on computer" , "Cause of computer failure?" .)