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