Rambling partner once joined up (9)
I believe the answer is:
excursive
'rambling' is the definition.
(I know that rambling can be written as excursive)
'partner once joined up' is the wordplay.
'partner once' becomes 'ex' (an ex-partner).
'joined up' becomes 'cursive'.
'ex'+'cursive'='EXCURSIVE'
(Other definitions for excursive that I've seen before include "Rambling" , "tendency to digress" , "Tending to digress" .)