Expert - for a doctor nothing to it (6)
I believe the answer is:
adroit
'expert' is the definition.
(adroit can mean expert or masterful)
'a doctor nothing to it' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'nothing' becomes 'O' (looks like zero - 0).
'to' says to put letters next to each other.
'a'+'dr'+'o'+'it'='ADROIT'
'for' is the link.
(Other definitions for adroit that I've seen before include "Deft and skilful" , "Quick or skilful" , "skill" , "Quick-witted" , "Deft and proficient" .)