One percent, upset with temperature, head off (9)
I believe the answer is:
intercept
'head off' is the definition.
'one percent upset with temperature' is the wordplay.
'one' becomes 'i' (roman numeral).
'upset with' indicates an anagram.
'temperature' becomes 't' (abbreviation).
'percent' mixed up with 't' can make 'ntercept'.
'i'+'ntercept'='INTERCEPT'
(Other definitions for intercept that I've seen before include "Obstruct to prevent reaching a destination" , "Stop in transit" , "Arrest" , "point on graph" , "Head off; ambush" .)