Wondered at five horse pens turning hollow (9)

I believe the answer is:

marvelled

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

'wondered' is the definition.
The definition and answer can be both to do with communicating as well as being past participle verbs.
Perhaps they are linked in a way I don't understand?

'five horse pens turning hollow' is the wordplay.
'five horse pens' becomes 'marve' (I can't justify this - if you can you should believe this answer much more).
'turning' says the letters should be written backwards.
'hollow' becomes 'dell'' (dell is a kind of hollow).
'dell' reversed gives 'lled'.
'marve'+'lled'='MARVELLED'

'at' acts as a link.

Can you help me to learn more?

(Other definitions for marvelled that I've seen before include "Was filled with wonder" , "was amazed" , "stood in awe" .)

I've seen this clue in the Sydney Morning Herald.

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.