Top time for successful mountaineering expeditions? (4,6)

I believe the answer is:

peak period

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

'successful mountaineering expeditions?' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'top time' is the wordplay.
I cannot really understand how this works, but
'top' could be 'peak' (I've seen this before) and 'peak' is located in the answer.
'time' could be 'period' (period is a kind of time) and 'period' is found within the answer.
No letters remain.
This may be the basis of clue (or it may be nonsense).

'for' acts as a link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.