Snack has section with additive (4)
I believe the answer is:
bite
'snack' is the definition.
(I know that snack can be written as bite)
'section with additive' is the wordplay.
'section' becomes 'bit' (both can mean part).
'with additive' becomes 'e' (I can't justify this - if you can you should believe this answer much more).
'bit'+'e'='BITE'
'has' acts as a link.
Can you help me to learn more?
(Other definitions for bite that I've seen before include "Small amount to eat" , "Nip; nibble" , "Puncture by 9 [ADDER]" , "power" , "(Of fish) take the bait" .)