Just tucked into additional small snack (9)
I believe the answer is:
elevenses
'snack' is the definition.
(I know that Elevenses is a type of snack)
'just tucked into additional small' is the wordplay.
'just' becomes 'even'.
'tucked into' is an insertion indicator.
'additional' becomes 'else' (something else is something additional).
'small' becomes 's' (abbreviation - e.g. clothes size).
'even' going inside 'else' is 'elevense'.
'elevense'+'s'='ELEVENSES'
(Other definitions for elevenses that I've seen before include "Morning snack" , "Early drink" , "Tea and biscuits, perhaps" , "Coffee time?" , "energy getting second break" .)