Climbers compete in Iceland (5)
I believe the answer is:
ivies
'climbers' is the definition.
(ivies are climbing plants)
'compete in iceland' is the wordplay.
'compete' becomes 'vie' (synonyms).
'in' is an insertion indicator.
'iceland' becomes 'is'.
'vie' going within 'is' is 'IVIES'.
(Other definitions for ivies that I've seen before include "Vines with evergreen leaves" , "Climbing plants with evergreen leaves" , "Climbers" .)