Puff from old trains? (6)
I believe the answer is:
pastry
'puff' is the definition.
(thesaurus)
'old trains?' is the wordplay.
'old' becomes 'past' (associated in meaning).
'trains?' becomes 'ry' (abbreviation for railway).
'past'+'ry'='PASTRY'
'from' is the link.
(Other definitions for pastry that I've seen before include "Dough of flour, water and shortening" , "croissant?" , "Piecrust; small cake" , "Foodstuff flattened with a rolling pin" , "Cooked dough" .)