Whistleblower is on top of computing overhaul (5)
I believe the answer is:
refit
'overhaul' is the definition.
(I've seen this before)
'whistleblower is on top of computing' is the wordplay.
'whistleblower' becomes 'ref' (referee).
'is on top of' means one lot of letters go next to another.
'computing' becomes 'it' (IT is an abbreviation for Information Technology).
'ref'+'it'='REFIT'
(Other definitions for refit that I've seen before include "Update" , "Do repair job" , "Replace ship's equipment" , "Process of improving equipment" , "Replace or repair equipment as part of a general overhaul" .)