Later, he’s active close to outskirts of Whitehall (5,3)
I believe the answer is:
night owl
'later he's active' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'close to outskirts of whitehall' is the wordplay.
'close' becomes 'nigh' (similar in meaning).
'outskirts of' means to remove the middle letters (only letters on the outskirts).
'whitehall' with its centre taken out is 'wl'.
'nigh'+'to'+'wl'='NIGHT-OWL'
Can you help me to learn more?
(Other definitions for night owl that I've seen before include "One preferring to work when it's dark" , "Person active in the small hours" , "he may go to bed at last" , "One likes to be active late into the evening" , "Person and creature active after dark" .)