Boy harbours desire to be a manual operator (7)
I believe the answer is:
surgeon
'a manual operator' is the definition.
(someone who operates with their hands ie manually)
'boy harbours desire' is the wordplay.
'boy' becomes 'son' (synonyms).
'harbours' means one lot of letters goes inside another.
'desire' becomes 'urge' (I've seen this before).
'son' enclosing 'urge' is 'SURGEON'.
'to be' acts as a link.
(Other definitions for surgeon that I've seen before include "Naval medic" , "9 practitioner (through keyhole?)" , "Operating medical practitioner" , "Sawbones" , "one performing in the theatre" .)