Headcase swaps sides – it's something of a cliché (8)
I believe the answer is:
chestnut
'cliche' is the definition.
(thesaurus)
'headcase swaps sides it's something' is the wordplay.
I cannot quite see how this works, but
'headcase' could be 'nut' (I've seen this before) and 'nut' is located in the answer.
'it's' could be 'es' ('e' can mean 'electronic' which is similar to 'IT') and 'es' is found in the answer.
This explanation may well be incorrect...
'of a' is the link.
Can you help me to learn more?
(Other definitions for chestnut that I've seen before include "Deep reddish-brown" , "Tree; old joke (colloq.)" , "Brown horse" , "Stale joke or cliche" , "Golden to reddish brown colour" .)