Clue: Deplore being endlessly cruel (6)
Answer: REGRET
Explanation: Deplore being endlessly cruel (6)
Chambers gives for **deplore**: to feel or express deep grief for, to express strong disapproval or disgust at, to give up as hopeless. The second meaning is the one I would normally attach to the word, but the first is required here; the wordplay is '[c]rue[l]' minus its outer letters ('endlessly').