Full development of fancy atrium they gutted (8)
I believe the answer is:
maturity
'full development' is the definition.
(I've seen this before)
'fancy atrium they gutted' is the wordplay.
'fancy' is an anagram indicator.
'gutted' means to remove the middle letters (to gut something is to remove its insides).
'they' with its middle taken out is 'ty'.
'atrium' with letters rearranged gives 'maturi'.
'maturi'+'ty'='MATURITY'
'of' acts as a link.
(Other definitions for maturity that I've seen before include "age" , "State of having reached full development" , "Ripeness" , "State of full growth or adulthood" , "Fool" .)