Crossword Genius

On safe tour, must swim. But energy wanes (4,3,2,5)

Ross

I believe the answer is:

runs out of steam

I'm a little stuck... Click here to teach me more about this clue!

'energy wanes' is the definition.
I know nothing about this answer so I can't tell whether this works.

'on safe tour must swim' is the wordplay.
'swim' indicates anagramming the letters.
'on'+'safe'+'tour'+'must'='onsafetourmust'
'onsafetourmust' is an anagram of 'RUNS OUT OF STEAM'.

'but' acts as a link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.
Want a hint initially instead of a full solution? Install my app