Unit in preliminary retreat (4)
I believe the answer is:
mile
'unit' is the definition.
(I know that mile is a type of linear measure)
'in preliminary retreat' is the wordplay.
'in' indicates the answer is hidden within the clue.
'retreat' shows that the letters should be reversed in order.
'elim' is hidden within 'preliminary'.
'elim' written backwards gives 'MILE'.
(Other definitions for mile that I've seen before include "'1,760 yards (4)'" , "5280 feet" , "'A miss is as good as a ....'" , "Unit of distance" , "Measure" .)