To growl at an egghead is the wrong thing to do (3)
I believe the answer is:
err
'the wrong thing to do' is the definition.
I can't tell whether this definition defines the answer.
'to growl at an egghead' is the wordplay.
'to growl' becomes 'rr' (I can't explain this - if you can you should believe this answer much more).
'at' says to put letters next to each other (I've seen this in other clues).
'an egghead' becomes 'e' (head letter of 'egg').
'rr' after 'e' is 'ERR'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for err that I've seen before include "Transgress" , "Bungle" , "Slip up" , "Make mistake in Canberra" , "Deviate" .)