Distance, not time - simple (4)
I believe the answer is:
mere
'simple' is the definition.
(synonyms)
'distance not time' is the wordplay.
'distance' becomes 'metre' (unit of distance).
'not' indicates named letters should be taken away.
'time' becomes 't' (abbreviation).
'metre' with 't' removed is 'MERE'.
(Other definitions for mere that I've seen before include "Without additions or modifications" , "Previously pure" , "Unimpressive" , "Pierre's mother identified" , "Pond - pure and simple" .)