Clue: To ensure sufficient funds remain below interest (9)
Answer: UNDERDRAW
Explanation: Definition: To ensure sufficient funds remain
UNDER (below) + DRAW (something interesting). Chambers only gives the definitions "To draw or describe with moderation or reticence or short of the truth; To cover the underside of with boards or lath and plaster", but online sources also define it as a kind of opposite of the more familiar "overdraw"