Help with harvest? Not all Irish do slapdash work (3,7)
I believe the answer is:
cut corners
'do slapdash work' is the definition.
(to cut corners is to work slapdashly)
'help with harvest? not all irish' is the wordplay.
'help with harvest?' becomes 'cutcorn' (cutting corn in a field).
'not all' means to remove the last letter (not quite all of it).
'irish' becomes 'erse' (I've seen this before).
'erse' with its last letter taken away is 'ers'.
'cutcorn'+'ers'='CUT CORNERS'
(Other definitions for cut corners that I've seen before include "Skimp on effort" , "Miss out things to save money" .)