Returner of questionnaire gloomy after change at top (10)
I believe the answer is:
respondent
'returner of questionnaire' is the definition.
'gloomy after change at top' is the wordplay.
I cannot really understand how this works, but
'change' could be 'dent' (dent is a kind of change) and 'dent' is located in the answer.
'at' could be 'on' (location) and 'on' is found in the answer.
This explanation may well be incorrect...
(Other definitions for respondent that I've seen before include "he defends suit" , "One in court" , "Person surveyed" , "defendant" , "Someone questioned" .)