Moderate frequently behind introduction of socialism (6)
I believe the answer is:
soften
'moderate' is the definition.
(I know that moderate can be written as soften)
'frequently behind introduction of socialism' is the wordplay.
'frequently' becomes 'often' (synonyms).
'behind' means one lot of letters go next to another.
'introduction of' says to take the initial letters.
The first letter of 'socialism' is 's'.
'often' after 's' is 'SOFTEN'.
(Other definitions for soften that I've seen before include "Tenderise" , "Become gentler" , "Mitigate" , "Make more plastic" , "Make less firm" .)