He does nothing but just tick over? (5)
I believe the answer is:
idler
'he does nothing but' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'tick over?' is the wordplay.
I cannot quite understand how this works, but
'tick over?' could be 'idle' (I've seen this before) and 'idle' is found within the answer.
A single letter 'r' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'just' is the link.
Can you help me to learn more?
(Other definitions for idler that I've seen before include "A lazy loafer" , "One resting" , "One who doesn't want to work" , "Bum" , "comparatively shoddy?" .)