Once, quietly, permit setter's padding word (9)

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

EX (once) P (quietly) LET (permit) I’VE (setter’s as in setter has). We had to check this as we were not aware that an ‘expletive’ can be a word included merely to fill a gap, or to fill up a sentence or line in verse.