He ensures he has a nap standing up! (6)
I believe the answer is:
teaser
'ensures he has a nap standing up' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he' is the wordplay.
I cannot really see how this works, but
'he' could be 'ase' and 'ase' is found in the answer.
The remaining letters 'ter' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for teaser that I've seen before include "One who irritates playfully" , "inducement" , "Tricky question; intriguing preliminary advert" , "one using a backcomb" , "Difficult question" .)