Be abusive to black-hearted engineers (6)
I believe the answer is:
revile
'be abusive' is the definition.
I can't judge whether this defines the answer.
'black-hearted engineers' is the wordplay.
'black' becomes 'evil' (similar in meaning).
'hearted' means one lot of letters goes inside another (some letters go to the heart of the word).
'engineers' becomes 're' (Royal Engineers).
'evil' put into 're' is 'REVILE'.
'to' acts as a link.
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" .)