Partner no longer going through dry set book (4)
I believe the answer is:
text
'set book' is the definition.
(eg text set for a class)
'partner no longer going through dry' is the wordplay.
'partner no longer' becomes 'ex' (ex-partner).
'going through' indicates putting letters inside.
'dry' becomes 'tt' (short for teetotal - no alcohol).
'ex' inserted within 'tt' is 'TEXT'.
(Other definitions for text that I've seen before include "Script" , "Written document" , "Written matter" , "inspiration for sermon" , "Use phone" .)