First two climbers on the tallest mountain were smartest (9)
I believe the answer is:
cleverest
'smartest' is the definition.
(I've seen this before)
'first two climbers on the tallest mountain were' is the wordplay.
I cannot really see how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
The remaining letters 'cleves' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cleverest that I've seen before include "of superlative brilliance" , "Smartest" , "most able" , "Brighter than others" .)