Protected by a climber, one was in contention (5)
I believe the answer is:
ivied
'protected by a climber' is the definition.
(ivy is a climbing plant)
'one was in contention' is the wordplay.
'one' becomes 'i' (Roman numeral).
'was in contention' becomes 'vied'.
'i'+'vied'='IVIED'
(Other definitions for ivied that I've seen before include "Being climbed all over, naturally" , "climber on outside of building" , "Covered with climbing plant" , "Covered with creeper" , "Decked with evergreen" .)