Let it remain, determined to save time (4)
I believe the answer is:
stet
'let it remain' is the definition.
(I've seen this before)
'determined to save time' is the wordplay.
'determined' becomes 'set' (synonyms).
'to save' is an insertion indicator.
'time' becomes 't' (abbreviation).
'set' going around 't' is 'STET'.
(Other definitions for stet that I've seen before include "Notation meaning as is, thus" , "Correction-reversing mark" , "Retain this type" , "Ignore correction" , "Let it stand (printer's correction)" .)