Sounds coming from reactionary and half-hearted session (6)
I believe the answer is:
noises
'sounds coming from reactionary' is the definition.
'noises' can be an answer for 'sounds' (noising is a kind of sounding). I am not certain of the 'coming from reactionary' bit.
'and half-hearted session' is the wordplay.
'and' becomes 'n' (common abbreviation for 'and').
'half' becomes 'oise' (I can't justify this - if you can you should give a lot more credence to this answer).
'hearted' indicates the central letters.
The middle of 'session' is 's'.
'n'+'oise'+'s'='NOISES'
(Other definitions for noises that I've seen before include "- - Off (stage farce)" , "Loud or harsh sounds" , "Sounds, rackets" , "Spreads abroad with sounds" , "Conventional phrases" .)