Champagne’s outside in case with a single ale, almost heavenly! (9)

'heavenly' is the definition.
(I know that heavenly can be written as celestial)

'champagne's outside in case with a single ale almost' is the wordplay.
'outside' suggests removing the centre.
'in case' becomes 'lest' (eg 'lest we forget').
'with' is a charade indicator (letters next to each other).
'a single' becomes 'i' (one).
'almost' means to remove the last letter (most of the word but not all of it).
'champagne' with its centre taken out is 'ce'.
'ale' with its last letter taken off is 'al'.
'ce'+'lest'+'i'+'al'='CELESTIAL'