Sang at assembly, not disheartened and not moving (8)
I believe the answer is:
stagnant
'not moving' is the definition.
(I've seen this before)
'sang at assembly not disheartened' is the wordplay.
'assembly' indicates anagramming the letters.
'disheartened' suggests removing the centre (the middle or heart is removed).
'not' with its centre removed is 'nt'.
'sang'+'at'='sangat'
'sangat' anagrammed gives 'stagna'.
'stagna'+'nt'='STAGNANT'
'and' acts as a link.
(Other definitions for stagnant that I've seen before include "Unmoving (water)" , "(Water) not flowing" , "Without vitality" , "(Of water) motionless" , "Moribund" .)