Hillary, say, ultimately athletic and agile (7)
I believe the answer is:
climber
'hillary' is the definition.
Both the answer and definition are singular nouns.
Maybe they are linked in a way I don't understand?
'ultimately athletic and agile' is the wordplay.
'ultimately' indicates one should take the final letters.
'and' says to put letters next to each other.
'agile' becomes 'limber' (I've seen this before).
The final letter of 'athletic' is 'c'.
'c'+'limber'='CLIMBER'
'say' is the link.
Can you help me to learn more?
(Other definitions for climber that I've seen before include "Edward Whymper, for example" , "Plant such as ivy" , "in the garden" , "Mountaineer" , "Plant that grows up a support" .)