Almost twelve taking lift became unconscious (5,3)

'became unconscious' is the definition.
(to doze off is to fall asleep)

'almost twelve taking lift' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'twelve' becomes 'dozen' (synonyms).
'taking' is a charade indicator (letters next to each other).
'lift' becomes 'doff' (as in to doff one's hat).
'dozen' with its last letter taken away is 'doze'.
'doze'+'doff'='DOZED OFF'