Explosive noise over harbour (6)
I believe the answer is:
report
'explosive noise' is the definition.
(I've seen this before)
'over harbour' is the wordplay.
'over' becomes 're'.
'harbour' becomes 'port' (I've seen this before).
're'+'port'='REPORT'
(Other definitions for report that I've seen before include "Make a complaint about (someone)" , "Porter (anag.)" , "Make a formal complaint" , "Formal document" , "Announce; rumour" .)