Brief flash in long cloud, measure dropping straight down (9)

'measure dropping straight down' is the definition.
(a plumbline measures depths)

'brief flash in long cloud' is the wordplay.
'brief' means to remove the last letter (a shorter or briefer version of the word).
'flash' becomes 'blink' (both can mean a very short amount of time).
'in' means one lot of letters goes inside another.
'long cloud' becomes 'plume' (eg a plume of smoke).
'blink' with its final letter taken away is 'blin'.
'blin' put inside 'plume' is 'PLUMBLINE'.