Speak at length about detail (6)
I believe the answer is:
dilate
'speak at length' is the definition.
(I've seen this before)
'about detail' is the wordplay.
'about' is an anagram indicator.
'detail' is an anagram of 'DILATE'.
(Other definitions for dilate that I've seen before include "Widen in detail as pupils may" , "Become wider or larger as eye pupils may" , "Enlarge or widen, like pupils perhaps" , "Widen, distend" , "Become wider or more open" .)