The first doctor is an idler (5)
I believe the answer is:
drone
'idler' is the definition.
(I know that drone is a type of idler)
'the first doctor is an' is the wordplay.
'the first' says to put letters next to each other.
'doctor' becomes 'Dr' (abbreviation).
'is an' becomes 'one'.
'dr' next to 'one' is 'DRONE'.
(Other definitions for drone that I've seen before include "fine lady?" , "Someone who lives on the labour of others" , "Non-worker" , "boring speaker" , "Speak long and tediously" .)