Speed maintained by vessel in intervals (6)
I believe the answer is:
spaces
'intervals' is the definition.
(I know this)
'speed maintained by vessel' is the wordplay.
'speed' becomes 'pace' (pace is a kind of speed).
'maintained by' means one lot of letters goes inside another (maintain can mean to keep hold of).
'vessel' becomes 'SS' (short for steamship).
'pace' put inside 'ss' is 'SPACES'.
'in' acts as a link.
(Other definitions for spaces that I've seen before include "cleared areas" , "boxes" , "Intervals" , "Blank areas" , "Empty areas" .)