Ruthlessly ambitious person using sleight horrifies you at first (5)
I believe the answer is:
pushy
'ruthlessly ambitious' is the definition.
'pushy' can be an answer for 'ambitious' (similar in meaning). I'm not certain of the 'ruthlessly' bit.
'person using sleight horrifies you at first' is the wordplay.
'at first' suggests taking the first letters.
The first letters of 'person using sleight horrifies you' is 'PUSHY'.
Can you help me to learn more?
(Other definitions for pushy that I've seen before include "Overly assertive" , "Informally aggressive, not patient" , "Excessively self-assertive" , "Forward" , "Aggressive; overbearing" .)