Former partner blocks dry electronic message (4)
I believe the answer is:
text
'electronic message' is the definition.
(I've seen this before)
'former partner blocks dry' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'blocks' indicates putting letters inside (upper walls off glass inside formal).
'dry' becomes 'tt' (abbreviation for teetotal).
'ex' placed into 'tt' is 'TEXT'.
(Other definitions for text that I've seen before include "Printed or written words" , "Words of something printed" , "Wording" , "Printed matter" , "Main body of book or page" .)