Undone by puff on crack (6)
I believe the answer is:
adrift
'undone' is the definition.
(thesaurus)
'puff on crack' is the wordplay.
'puff' becomes 'ad'.
'on' is a charade indicator (letters next to each other).
'crack' becomes 'rift' (rift is a kind of crack**).
'ad'+'rift'='ADRIFT'
'by' is the link.
(Other definitions for adrift that I've seen before include "Purposeless" , "Unanchored - lost" , "Floating freely, not moored" , "(Of ship) out of control" , "without guidance?" .)