I'd returned behind time to speak at length (6)
I believe the answer is:
dilate
'speak at length' is the definition.
(I've seen this before)
'i'd returned behind time' is the wordplay.
'returned' is a reversal indicator.
'behind time' becomes 'late' (I've seen this before).
'id' in reverse letter order is 'di'.
'di'+'late'='DILATE'
'to' is the link.
(Other definitions for dilate that I've seen before include "Speak or write at length - widen" , "Widen like eye pupils" , "Enlarge or widen, like pupils perhaps" , "Make or become wider" , "Make wider or larger" .)