Be abusive to black-hearted engineers (6)
I believe the answer is:
revile
'be' is the definition.
Although both the answer and definition are verbs in their base form, I don't see how they can define each other.
'abusive to black-hearted engineers' is the wordplay.
'abusive' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'to black' becomes 'evil' ('evil' can be similar in meaning to 'black').
'hearted' says to take the centre.
'engineers' becomes 'res'.
The middle of 'res' is 'e'.
'r'+'evil'+'e'='REVILE'
Can you help me to learn more?
(Other definitions for revile that I've seen before include "Abuse verbally" , "Criticise - abuse" , "Run down" , "Criticise in abusive language" , "Vilify" .)