They suffer informal and formal attire (10)
I believe the answer is:
casualties
'they suffer' is the definition.
'informal and formal attire' is the wordplay.
'informal' becomes 'casual' ('casual' can be similar in meaning to 'informal').
'and' means one lot of letters go next to another.
'formal attire' becomes 'ties'.
'casual'+'ties'='CASUALTIES'
(Other definitions for casualties that I've seen before include "Victims" , "Injured people" .)