Magnificent reservoir finished up unserviceable (9)
I believe the answer is:
sumptuous
'magnificent' is the definition.
(I've seen this in another clue)
'reservoir finished up unserviceable' is the wordplay.
'reservoir' becomes 'sump' (sump is a kind of reservoir).
'finished' becomes 'out' ('I'm out' means 'I'm finished').
'up' is a reversal indicator (in down clue: letters go upwards).
'unserviceable' becomes 'us' (abbreviation).
'out' written backwards gives 'tuo'.
'sump'+'tuo'+'us'='SUMPTUOUS'
(Other definitions for sumptuous that I've seen before include "Luxurious" , "Grand" , "Magnificent" , "Delicious and expensive" , "Extravagant" .)