Important judge participating in climbing range (5)
I believe the answer is:
major
'important' is the definition.
(major can mean of great importance)
'judge participating in climbing range' is the wordplay.
'judge' becomes 'J' (abbreviation for judge).
'participating in' is an insertion indicator.
'climbing' is a reversal indicator (in a down clue, letters climb up).
'range' becomes 'roam' (synonyms).
'roam' in reverse letter order is 'maor'.
'j' inserted within 'maor' is 'MAJOR'.
(Other definitions for major that I've seen before include "Very significant - army officer" , "Chief - officer" , "Of greater importance or stature" , "Conservative PM" , "Uppermost" .)