Extremely speedy gesture of assent in Church council (5)
I believe the answer is:
synod
'church council' is the definition.
(I've seen this before)
'extremely speedy gesture of assent' is the wordplay.
'extremely' says to hollow out the word (remove centre letters).
'gesture of assent' becomes 'nod' (I've seen this before).
'speedy' with its centre removed is 'sy'.
'sy'+'nod'='SYNOD'
'in' is the link.
(Other definitions for synod that I've seen before include "council meeting" , "Ecclesiastical council" , "Clerical assembly" , "Forum for discussing" , "Council of bishops" .)