Weary nightwatchman's last shifts, one obviously tired (6)
I believe the answer is:
yawner
'one obviously tired' is the definition.
I can't tell whether this definition defines the answer.
'weary nightwatchman's last shifts' is the wordplay.
'last' indicates one should take the final letters.
'shifts' is an anagram indicator.
The final letter of 'nightwatchman' is 'n'.
'weary'+'n'='wearyn'
'wearyn' is an anagram of 'YAWNER'.
Can you help me to learn more?
(Other definitions for yawner that I've seen before include "Person giving oral evidence of boredom" , "he might well gape" .)