Affected part of week after returning (4)
I believe the answer is:
fake
'affected' is the definition.
(thesaurus)
'part of week after returning' is the wordplay.
'part of' indicates a hidden word.
'returning' shows that the letters should be reversed in order.
'ekaf' can be found hidden inside 'week after'.
'ekaf' written backwards gives 'FAKE'.
(Other definitions for fake that I've seen before include "Artificial" , "False, sham" , "Fabricate" , "Forge(ry)" , "Bogus" .)