Assume rep upset nurse (7)
I believe the answer is:
pretend
'assume' is the definition.
(I know this)
'rep upset nurse' is the wordplay.
'upset' indicates an anagram.
'nurse' becomes 'tend' (tend can mean to nurse or look after).
'rep' anagrammed gives 'pre'.
'pre'+'tend'='PRETEND'
(Other definitions for pretend that I've seen before include "Sham" , "Imagine" , "Imagined" , "Suppose" , "Simulated" .)