Aunt mad for sandwich filling (4)
I believe the answer is:
tuna
'sandwich filling' is the definition.
(tuna is a common sandwich filling)
'aunt mad' is the wordplay.
'mad' indicates anagramming the letters.
'aunt' with letters rearranged gives 'TUNA'.
'for' is the link.
(Other definitions for tuna that I've seen before include "swimmer" , "Sea creature" , "A fish; sounds like piano juster" , "Large food fish" , "Tunny" .)