Snack, though tiny, lacking filling (5)
I believe the answer is:
butty
'snack' is the definition.
(filled or open sandwich)
'though tiny lacking filling' is the wordplay.
'though' becomes 'but'.
'lacking filling' says to hollow out the word (remove centre letters).
'tiny' with its centre taken out is 'ty'.
'but'+'ty'='BUTTY'
(Other definitions for butty that I've seen before include "Sandwich; freight barge" , "Partner" , "something to eat" , "Sandwich; towed barge" .)