Purchase scrap of cloth to plug dyke (8)
I believe the answer is:
leverage
'purchase' is the definition.
(I know that purchase can be written as leverage)
'scrap of cloth to plug dyke' is the wordplay.
'scrap of cloth' becomes 'rag' (I've seen this before).
'to plug' indicates putting letters inside (to plug can mean to block a gap within something).
'dyke' becomes 'levee' (synonyms).
'rag' put inside 'levee' is 'LEVERAGE'.
(Other definitions for leverage that I've seen before include "can achieve power" , "Economic clout" , "What crowbar provides" , "Mechanical advantage; influence" , "Power to influence" .)