New measure of computer storage filling two clouds (5)
I believe the answer is:
nimbi
'clouds' is the definition.
(nimbus is a kind of cloud)
'new measure of computer storage filling two' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'measure of computer storage' becomes 'MB' (megabytes).
'filling' is an insertion indicator.
'two' becomes 'II' (Roman numerals).
'mb' inserted into 'ii' is 'imbi'.
'n'+'imbi'='NIMBI'
(Other definitions for nimbi that I've seen before include "Rain clouds - halos" , "Rain-bearing clouds" , "rings" , "Dark things" , "Rain-clouds" .)