Doctor takes on a practice, a very timid person (5)
I believe the answer is:
mouse
'timid person' is the definition.
(I've seen this in another clue)
'doctor takes on a practice' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'takes on' means one lot of letters go next to another.
'a practice' becomes 'use' (use is a kind of practice).
'mo'+'use'='MOUSE'
'a very' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for mouse that I've seen before include "Small verminous rodent" , "Small creature" , "Small rodent - computer device" , "Squeaky creature" , "See 1 down" .)