It's having clothes on that's tiresome (7)
I believe the answer is:
wearing
'it's having' is the definition.
(wearing is a kind of having)
'clothes on that's tiresome' is the wordplay.
I cannot quite understand how this works, but
'clothes' could be 'wear' (synonyms) and 'wear' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for wearing that I've seen before include "Irksome" , "sporting" , "Trying" .)