Council puts unknown knight in ground (5)
I believe the answer is:
synod
'council' is the definition.
(synod is a kind of council)
'unknown knight in ground' is the wordplay.
'unknown' becomes 'y' (algebra).
'knight' becomes 'N' (abbreviation for knight in chess).
'in' is an insertion indicator.
'ground' becomes 'sod' (sod is a kind of ground).
'y'+'n'='yn'
'yn' going into 'sod' is 'SYNOD'.
'puts' acts as a link.
(Other definitions for synod that I've seen before include "Forum for discussing" , "council meeting" , "group of clergymen" , "Ecclesiastical assembly" , "Ecclesiastical council" .)