Vital during day teachers should get a little food (6,3)
I believe the answer is:
monkey nut
'a little food' is the definition.
(small food item)
'vital during day teachers' is the wordplay.
'vital' becomes 'key' (I've seen this before).
'during' means one lot of letters goes inside another.
'day' becomes 'mon' (abbreviation for Monday).
'teachers' becomes 'nut' (National Union of Teachers).
'mon'+'nut'='monnut'
'key' going within 'monnut' is 'MONKEY NUT'.
'should get' acts as a link.
(Other definitions for monkey nut that I've seen before include "It could be in the bag" , "goober" , "Not my nuke (anag.)" , "source of oil" , "seed" .)