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