Medium blocking conscious feeling (9)
I believe the answer is:
sentiment
'feeling' is the definition.
(sentiment is a kind of feeling)
'medium blocking conscious' is the wordplay.
'medium' becomes 'm' (as in UK clothing sizes).
'blocking' is an insertion indicator.
'conscious' becomes 'sentient' (similar in meaning).
'm' put into 'sentient' is 'SENTIMENT'.
(Other definitions for sentiment that I've seen before include "Emotional thought" , "Emotional feeling" , "judgement" , "Mawkishness" , "View, opinion, feeling" .)