Lest, perhaps, the hand coming through should frighten you (7)
I believe the answer is:
startle
'frighten you' is the definition.
'startle' can be an answer for 'frighten' (I've seen this before). I am not sure about the 'you' bit.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for startle that I've seen before include "Surprise with starlet" , "Rattles (anag.)" , "Shock (someone)" , "Alarm (someone)" , "Frighten suddenly" .)