Snack otherwise includes cool seconds (9)
I believe the answer is:
elevenses
'snack' is the definition.
(I know that Elevenses is a type of snack)
'otherwise includes cool seconds' is the wordplay.
'otherwise' becomes 'else' (synonyms).
'includes' is an insertion indicator.
'cool' becomes 'even' (I can't justify this - if you can you should believe this answer much more).
'seconds' becomes 's'.
'else' placed around 'even' is 'elevense'.
'elevense'+'s'='ELEVENSES'
Can you help me to learn more?
(Other definitions for elevenses that I've seen before include "Coffee time?" , "Mid-morning break" , "energy getting second break" , "Morning snack" , "Tea and biscuits, perhaps" .)