I stress out nuns (7)
I believe the answer is:
sisters
'nuns' is the definition.
(I've seen this before)
'i stress out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'i'+'stress'='istress'
'istress' anagrammed gives 'SISTERS'.
(Other definitions for sisters that I've seen before include "Nuns - nurses - siblings" , "Family relatives" , "Aunts may be" , "Resists (anag)" , "Relatives in hospital?" .)