Regret starting running up Everest (3)
I believe the answer is:
rue
'regret' is the definition.
(I know this)
'starting running up everest' is the wordplay.
'starting' indicates taking the first letters.
The initial letters of 'running up everest' is 'RUE'.
(Other definitions for rue that I've seen before include "Regret or repent" , "Plant producing oil" , "Lament; a herb" , "Feel bad" , "Regret, be sorry for" .)