Tell us about the census (7)
I believe the answer is:
recount
'the census' is the definition.
(thesaurus)
'tell us about' is the wordplay.
I cannot really see how this works, but
'tell' could be 'count' (I've seen this before) and 'count' is found in the answer.
'about' could be 're' (common abbreviation for 'with reference to') and 're' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for recount that I've seen before include "Give report" , "Tell in detail" , "post-election procedure?" , "Communicate" , "Go over details" .)