The hiragana ends with an う sound, so you know this is a verb. To support pay is another way to say to pay.

支払う is a common word used when paying for something, such as a bill, fee, or purchase. It can be used in both casual and formal contexts, but it feels a bit more complete and proper than 払う. For example, in casual conversation you'd be more likely to say 私が払うよ (I'll pay), but you could use 支払う too — it would just sound slightly more polite.

This is just the on'yomi reading of 支 added on to 払う. You learned that reading with the kanji, so you should be okay here.