Former wife involved in dancing this reel (9)
I believe the answer is:
erstwhile
'former' is the definition.
('erstwhile' can be a synonym of 'former')
'wife involved in dancing this reel' is the wordplay.
'wife' becomes 'w'.
'involved in' indicates putting letters inside.
'dancing' is an anagram indicator.
'this'+'reel'='thisreel'
'thisreel' is an anagram of 'ersthile'.
'w' going within 'ersthile' is 'ERSTWHILE'.
(Other definitions for erstwhile that I've seen before include "Former - list where (anag)" , "Quondam" , "Former, previous" , "Formerly" , "In the past" .)