Moderate left infiltrating the centre (4)
I believe the answer is:
mild
'moderate' is the definition.
(similar in meaning)
'left infiltrating the centre' is the wordplay.
'left' becomes 'l' (common abbreviation).
'infiltrating' is an insertion indicator.
'the centre' becomes 'mid' ().
'l' put within 'mid' is 'MILD'.
(Other definitions for mild that I've seen before include "old cardinals ..." , "Meek" , "Far from extreme" , "Calm" , "Gentle - bland" .)