Children observe deputy head (4)
I believe the answer is:
seed
'children' is the definition.
(I've seen this before)
'observe deputy head' is the wordplay.
'observe' becomes 'see' (I've seen this before).
'head' suggests taking the first letters.
The initial letter of 'deputy' is 'd'.
'see'+'d'='SEED'
(Other definitions for seed that I've seen before include "one with potential to grow up?" , "Germ; top player" , "Top competitor" , "A plant grows from it" , "people decline to go to" .)