Animal helper gets centre of chocolate block (4)
I believe the answer is:
veto
'block' is the definition.
(to veto is to block a decision or action)
'animal helper gets centre of chocolate' is the wordplay.
'animal helper' becomes 'vet' (I can't justify this - if you can you should believe this answer much more).
'gets' says to put letters next to each other.
'centre of' says to take the centre.
'chocolate' becomes 'hot chocolate' (synonyms).
The centre of 'hot' is 'o'.
'vet'+'o'='VETO'
Can you help me to learn more?
(Other definitions for veto that I've seen before include "Blocking vote" , "Refusal to assent" , "Rule out" , "Forbid; prohibition" , "Right to ban" .)