Particular deal -- it collapsed (6)
I believe the answer is:
detail
'particular' is the definition.
('detail' can be a synonym of 'particular')
'deal it collapsed' is the wordplay.
'collapsed' indicates an anagram.
'deal'+'it'='dealit'
'dealit' anagrammed gives 'DETAIL'.
(Other definitions for detail that I've seen before include "Describe fully, give particulars of" , "Relate minutely" , "Dilate with individual item" , "An individual item or element" , "Small item; assign" .)