Placid bishop collared by peer at end of debate (7)
I believe the answer is:
equable
'placid' is the definition.
(synonyms)
'bishop collared by peer at end of debate' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'collared by' is an insertion indicator (I've seen this in other clues).
'peer' becomes 'equal' (synonyms).
'at end of' suggests the final letters.
The final letter of 'debate' is 'e'.
'b' placed within 'equal' is 'equabl'.
'equabl'+'e'='EQUABLE'
(Other definitions for equable that I've seen before include "Uniform - even" , "Placid; unvarying" , "not easily rattled" , "Calm, even-tempered" , "Not extreme" .)