Ruffian wanting good doctor? One's on hand (5)
I believe the answer is:
thumb
'one's on hand' is the definition.
(a thumb is part of a hand)
'ruffian wanting good doctor?' is the wordplay.
'ruffian' becomes 'thug' (both can mean a violent person).
'wanting' indicates named letters should be taken away (to want something can mean to lack it or need it).
'good' becomes 'g' (abbreviation).
'doctor?' becomes 'MB' (abbreviation for Bachelor of Medicine).
'thug' with 'g' taken away is 'thu'.
'thu'+'mb'='THUMB'
(Other definitions for thumb that I've seen before include "Second part of 12" , "Indicate need for lift" , "could do it for a lift" , "Attempt to obtain a lift" , "Digit that enables grasping" .)