Snack? Still tucking into other source of sustenance (9)
I believe the answer is:
elevenses
'snack?' is the definition.
(I've seen this before)
'still tucking into other source of sustenance' is the wordplay.
'still' becomes 'even' ('even' can be a synonym of 'still'**).
'tucking into' is an insertion indicator.
'other' becomes 'else' ('else' can be a synonym of 'other').
'source of' suggests taking the first letters.
The first letter of 'sustenance' is 's'.
'even' inserted into 'else' is 'elevense'.
'elevense'+'s'='ELEVENSES'
(Other definitions for elevenses that I've seen before include "Morning tea or coffee break" , "Early drink" , "Mid-morning snacks" , "Coffee time?" , "Tea and biscuits, perhaps" .)