Head of Royal Arsenal discovered smuggler (6)
I believe the answer is:
runner
'smuggler' is the definition.
(synonyms)
'head of royal arsenal discovered' is the wordplay.
'head of' says to take the initial letters.
'arsenal' becomes 'gunnery' (both can mean a place guns are made or kept).
'discovered' indicates the central letters (dis-covered - outer letters removed).
The centre of 'gunnery' is 'unner'.
The initial letter of 'royal' is 'r'.
'r'+'unner'='RUNNER'
(Other definitions for runner that I've seen before include "Horse (in a race); bean plant" , "Kind of bean for an athlete" , "Messenger; plant shoot" , "done by con man, perhaps" , "Harrier" .)