This is a container clue - one word or phrase is inserted inside another word. Look for an indicator that suggests putting something 'in', 'inside', 'within', or similar.
'Aboard' is the container indicator. You need to find a word meaning 'belt' and put it inside a word meaning 'jet'. The definition 'Spend a lot of money' is at the start of the clue.
Answer: SPLASH OUT | LASH (belt) is inserted 'aboard' SPOUT (jet) to give SPLASH OUT (spend a lot of money)