Sandwich filling occasionally wet? Dear me! (4)
I believe the answer is:
edam
'sandwich filling' is the definition.
(edam might be used in a sandwich)
'occasionally wet? dear me' is the wordplay.
'occasionally' means one should take alternating letters.
'wet'+'dear'+'me'='wetdearme'
The alternating letters of 'wetdearme' are 'EDAM'.
(Other definitions for edam that I've seen before include "Dutch 8 across" , "milk product" , "Dutch specialty" , "Cheese made in Holland" , "Sphere of Dutch cheese" .)