One’s little more than extra restraint: pound retained by one making expenditure (9)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

BIT (a restraint for a horse), then L (£ = abbreviation for pound sterling) contained in PAYER (one making expenditure). Bit player = an actor who plays a very small part, but still more than an “extra” (or “background actor”) who doesn’t usually get anything to say.