Modify a flat over time (5)
I believe the answer is:
adapt
'modify' is the definition.
(to modify is to improve or adapt)
'a flat over time' is the wordplay.
'flat' becomes 'pad' (a person's home).
'over' says the letters should be written backwards.
'time' becomes 't' (abbreviation).
'pad' backwards is 'dap'.
'a'+'dap'+'t'='ADAPT'
(Other definitions for adapt that I've seen before include "Change to suit a different purpose" , "Modify; acclimatise" , "Become adjusted to new conditions" , "Convert to another use or situation" , "Adjust to new use" .)