Strikebreaker promises to pay for plant (8)
I believe the answer is:
scabious
'plant' is the definition.
'strikebreaker promises to pay' is the wordplay.
'strikebreaker' becomes 'scab' ('scab' can be a synonym of 'strikebreaker').
'promises to pay' becomes 'ious' (more than one iou).
'scab'+'ious'='SCABIOUS'
'for' acts as a link.
(Other definitions for scabious that I've seen before include "Bluish purple flower" , "Plant of the teasel family - covered in mange - so is Cuba (anag)" , "One blooming" , "Casio bus (anag) - plant" , "Wild flower" .)