Ridge veteran discovered in retirement (5)
I believe the answer is:
arete
'ridge' is the definition.
(arete is a kind of ridge)
'veteran discovered in retirement' is the wordplay.
'discovered' says the answer is hidden in the clue (remove the letters covering the answer in the clue).
'in retirement' says the letters should be written backwards (retire can mean to go back or backwards).
'etera' can be found hidden inside 'veteran'.
'etera' reversed gives 'ARETE'.
(Other definitions for arete that I've seen before include "feature of range" , "part of mountain range" , "Sharp ridge on mountain, with a tree" , "Glacial ridge" , "Narrow mountain ridge - eater (anag)" .)