Watch ruffian seizing man on board? Not right (7)
I believe the answer is:
lookout
'watch' is the definition.
('lookout' can be a synonym of 'watch')
'ruffian seizing man on board? not right' is the wordplay.
'ruffian' becomes 'lout' (I've seen this before).
'seizing' means one lot of letters goes inside another.
'man on board?' becomes 'rook' (chess).
'not' suggests deleting specific letters.
'right' becomes 'r' (common abbreviation).
'rook' with 'r' removed is 'ook'.
'lout' enclosing 'ook' is 'LOOKOUT'.
(Other definitions for lookout that I've seen before include "One keeping watch" , "Observatory" , "Watching person" , "Watchtower" , "Sentinel, watcher" .)