Brief time running banks (8)
I believe the answer is:
fleeting
'brief' is the definition.
(I've seen this before)
'time running banks' is the wordplay.
'time' becomes 't' (abbreviation).
'running' becomes 'fleeing' (running away).
'banks' indicates putting letters inside (bank can mean to surround).
't' going into 'fleeing' is 'FLEETING'.
(Other definitions for fleeting that I've seen before include "Passing quickly, transitory" , "Ephemeral" , "Evanescent" , "Transient" , "Momentary" .)