Bit of a thick skin that's required to contact the Guardian? (6)
I believe the answer is:
callus
'bit of a thick skin' is the definition.
(hard bit of skin)
'contact the guardian?' is the wordplay.
'contact' becomes 'call'.
'the guardian?' becomes 'us' (the crossword setter and their Guardian colleagues).
'call'+'us'='CALLUS'
'that's required to' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for callus that I've seen before include "A sore point" , "Thickened skin patch" , "Skin problem" , "Area of hard skin" , "This thick-skinned" .)