Official border moving north over time is theft (9)
I believe the answer is:
pilferage
'is theft' is the definition.
(I have seen 'Petty theft ' mean 'pilferage' so perhaps 'theft' could also mean 'pilferage')
'official border moving north over time' is the wordplay.
'official' becomes 'ref' (match official in football, say).
'border' becomes 'lip' (I've seen this in other clues).
'moving north' says the letters should be written backwards (in a down clue, letters go up or 'north').
'over' says to put letters next to each other (in a down clue, some letters go over others).
'time' becomes 'age' (an age is a period of time).
'ref'+'lip'='reflip'
'reflip' backwards is 'pilfer'.
'pilfer'+'age'='PILFERAGE'
(Other definitions for pilferage that I've seen before include "Thefts" , "Petty theft" , "Minor stealing" , "Theft (of small items)" .)