Watchman having to watch it! (7)
I believe the answer is:
lookout
'watchman' is the definition.
(I know that lookout is a type of watchman)
'to watch it' is the wordplay.
'to watch' becomes 'look' (I've seen this in another clue).
'it' becomes 'out' (I can't justify this - if you can you should believe this answer much more).
'look'+'out'='LOOKOUT'
'having' is the link.
Can you help me to learn more?
(Other definitions for lookout that I've seen before include "Observation point" , "Sentinel, watcher" , "Person on watch for dangers" , "Vantage point" , "Watching person" .)