Not liable, as formerly, to be almost empty (6)
I believe the answer is:
exempt
'not liable' is the definition.
(I've seen this in another clue)
'formerly to be almost empty' is the wordplay.
'formerly' becomes 'ex' (prefix meaning former or onetime).
'to be almost' means to remove the last letter (most of the word but not all of it).
'empty' with its last letter removed is 'empt'.
'ex'+'empt'='EXEMPT'
'as' is the link.
(Other definitions for exempt that I've seen before include "Not subject to or liable for" , "let off the hook" , "free from tax, say" , "Freed from liability (to tax etc)" , "Free from, eg, tax" .)