Swell girl departed (6)
I believe the answer is:
dilate
'swell' is the definition.
(I've seen this before)
'girl departed' is the wordplay.
'girl' becomes 'di' (**).
'departed' becomes 'late' (I've seen this before).
'di'+'late'='DILATE'
(Other definitions for dilate that I've seen before include "Widen as eyes may" , "Expand - talk at length" , "Widen in detail as pupils may" , "Speak or write at length - widen" , "Enlarge or widen, like pupils perhaps" .)