They obstruct gunners in boats (8)
I believe the answer is:
barrages
'they obstruct' is the definition.
The definition suggests an adverb but the answer is not.
'gunners in boats' is the wordplay.
'gunners' becomes 'RA' (Royal Artillery).
'in' indicates putting letters inside.
'boats' becomes 'barges' (barge is a kind of boat).
'ra' put within 'barges' is 'BARRAGES'.
Can you help me to learn more?
(Other definitions for barrages that I've seen before include "Rapid and continuous deliveries, of gunfire perhaps" , "Continuous showers of shell" , "Bombardments, onslaughts" .)