Rent out foxy clothes in serious fashion (7)
I believe the answer is:
sternly
'in serious fashion' is the definition.
(in a serious or stern manner)
'rent out foxy clothes' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'foxy' becomes 'sly' (synonyms).
'clothes' means one lot of letters goes inside another.
'rent' is an anagram of 'tern'.
'tern' put into 'sly' is 'STERNLY'.
(Other definitions for sternly that I've seen before include "In serious way" , "Cruelly" .)