Sandwich behind pasty, ultimately (5)
I believe the answer is:
butty
'sandwich' is the definition.
(I've seen this before)
'behind pasty ultimately' is the wordplay.
'behind' becomes 'butt' ('butt' can be a synonym of 'behind').
'pasty ultimately' becomes 'y' (I am not sure about this - if you are sure you should believe this answer much more).
'butt'+'y'='BUTTY'
(Other definitions for butty that I've seen before include "Sandwich; freight barge" , "Sandwich; towed barge" , "Partner" , "something to eat" .)