Hellish day I took part in last month (7)
I believe the answer is:
demonic
'hellish' is the definition.
('demonic' can be a synonym of 'hellish')
'day i took part in last month' is the wordplay.
'day' becomes 'mon'.
'took part in' means one lot of letters goes inside another.
'last month' becomes 'dec'.
'mon'+'i'='moni'
'moni' going within 'dec' is 'DEMONIC'.
(Other definitions for demonic that I've seen before include "Cruel like the devil" , "Dime con (anag.)" , "Like evil spirits" , "As if possessed by devil" , "Like a devil or evil spirit" .)