It might protect an illegal dealer (5)
I believe the answer is:
fence
'protect an illegal dealer' is the definition.
'fence' can be an answer for 'protect' (fencing is a kind of protecting). I am not sure about the 'an illegal dealer' bit.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for fence that I've seen before include "Dealer in stolen property" , "[SITTER] on this won't make a decision!" , "Railing" , "use blade" , "Enclosing barrier" .)