Came across reactionary for a moderate (6)
I believe the answer is:
temper
'moderate' is the definition.
(I know that moderate can be written as temper)
'came across reactionary for a' is the wordplay.
'came across' becomes 'met' (to come across is to meet or encounter).
'reactionary' says the letters should be written backwards (reactions happen in the opposite direction).
'for a' becomes 'per' ('per' can mean 'for each' person or 'for a' person).
'met' back-to-front is 'tem'.
'tem'+'per'='TEMPER'
(Other definitions for temper that I've seen before include "Tantrum" , "Mood, maybe angry" , "Harden metal by heating and cooling" , "bait" , "Volatile anger - don't lose it" .)