Overlook gen about displaced Celt (7)
I believe the answer is:
neglect
'overlook' is the definition.
(synonyms)
'gen about displaced celt' is the wordplay.
'about' shows that the letters should be reversed in order.
'displaced' indicates an anagram.
'gen' back-to-front is 'neg'.
'celt' is an anagram of 'lect'.
'neg'+'lect'='NEGLECT'
(Other definitions for neglect that I've seen before include "Failure to give proper care" , "Fail to look after (something)" , "Fail to take proper care of, be derelict" , "Disregard or fail to care for" , "Fail to notice or care for" .)