Up towards rear of boat, keeping watch (5)
I believe the answer is:
aloft
'up' is the definition.
(aloft means above or overhead)
'towards rear of boat keeping watch' is the wordplay.
'towards' becomes 'af' (I can't explain this - if you can you should give a lot more credence to this answer).
'rear of' says to take the final letters.
'keeping' is an insertion indicator.
'watch' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
The final letter of 'boat' is 't'.
'af'+'t'='aft'
'aft' going around 'lo' is 'ALOFT'.
Can you help me to learn more?
(Other definitions for aloft that I've seen before include "up the mast" , "in a high position" , "Skyward" , "At a considerable height" , "Heading for heaven" .)