One competed with climbers (5)
I believe the answer is:
ivied
'with climbers' is the definition.
(covered in ivy)
'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 creeper" , "climber on outside of building" , "Overgrown by a climbing evergreen" , "Being climbed all over, naturally" , "Decked with evergreen" .)