Maximum speed maintained by unit in the past (4)
I believe the answer is:
once
'in the past' is the definition.
(once upon a time)
'maximum speed maintained by unit' is the wordplay.
'maximum speed' becomes 'c' (I've seen this in other clues).
'maintained by' means one lot of letters goes inside another (maintain can mean to keep hold of).
'unit' becomes 'one' (synonyms).
'c' inserted inside 'one' is 'ONCE'.
(Other definitions for once that I've seen before include "One-time - one time" , "At some former time" , "In days gone by" , "in bygone era" , "Quick examination" .)