Wearing them, it upsets us to sit badly (5)
I believe the answer is:
suits
'wearing them it' is the definition.
I can't tell whether this defines the answer.
'upsets us to sit badly' is the wordplay.
'upsets' says the letters should be written backwards.
'badly' indicates anagramming the letters.
'us' written backwards gives 'su'.
'sit' with letters rearranged gives 'its'.
'su'+'its'='SUITS'
Can you help me to learn more?
(Other definitions for suits that I've seen before include "Hearts, clubs, diamonds, spades" , "Formal garments" , "cards" , "Becomes; formal wear" , "Businessman's" .)