Active amount of computer storage dropped in river (6)
I believe the answer is:
nimble
'active' is the definition.
(similar in meaning)
'amount of computer storage dropped in river' is the wordplay.
'amount of computer storage' becomes 'mb' (abbreviation for megabyte).
'dropped in' is an insertion indicator.
'river' becomes 'nile' (Nile River is an example).
'mb' inserted into 'nile' is 'NIMBLE'.
(Other definitions for nimble that I've seen before include "Swift, active" , "Quick on the feet" , "Moving quickly and lightly" , "'Jack be ......, Jack be quick' (6)." , "'Agile, spry (6)'" .)