Agent with all over tan is a boring tool! (6)
I believe the answer is:
trepan
'boring tool' is the definition.
(I've seen this in another clue)
'agent with all over tan is a' is the wordplay.
'agent' becomes 'rep' (short for representative).
'with' means one lot of letters go next to another.
'all over tan' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'is a' becomes 'an'.
'rep' after 't' is 'trep'.
'trep'+'an'='TREPAN'
Can you help me to learn more?
(Other definitions for trepan that I've seen before include "old saw" , "cut through bone" , "Cranial surgical instrument" , "Surgically create a hole in the skull" , "Make perforation in person's skull" .)