Gang, how clever you are cracking this! (6)
I believe the answer is:
clique
'gang' is the definition.
(thesaurus)
'clever you are cracking this' is the wordplay.
'clever you' becomes 'iq' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'are cracking' indicates putting letters inside (inserted letters go into a gap or crack).
'this' becomes 'clue' (the clue referring to itself!).
'iq' placed inside 'clue' is 'CLIQUE'.
'how' is the link.
Can you help me to learn more?
(Other definitions for clique that I've seen before include "Group hostile to outsiders" , "Inner group" , "set" , "Exclusive group of people" , "'Exclusive circle of people, hostile to outsiders (6)'" .)