One competed with climbers in prominent positions (5)
I believe the answer is:
ivied
'with climbers in prominent positions' is the definition.
(ivy is a type of climbing plant)
'one competed' is the wordplay.
'one' becomes 'I' (Roman numeral).
'competed' becomes 'vied' ('vie' can be a synonym of 'compete').
'i'+'vied'='IVIED'
(Other definitions for ivied that I've seen before include "Covered with climbing plant" , "a climber going up the wall" , "Being climbed all over, naturally" , "Overgrown by a climbing evergreen" , "Overgrown with creeper" .)