Thatcher's modernising successor firing son in due course (5)
I believe the answer is:
later
'in due course' is the definition.
(I've seen this before)
'thatcher's modernising successor firing son' is the wordplay.
'thatcher's modernising successor' becomes 'slater'.
'firing' is a deletion indicator.
'son' becomes 's'.
'slater' with 's' taken out is 'LATER'.
(Other definitions for later that I've seen before include "at a subsequent point" , "In due course" , "Tardier" , "More recent" , "goodbye" .)