Robot's hearts set on first instrumentalist (6)
I believe the answer is:
oboist
'instrumentalist' is the definition.
(oboist is a kind of instrumentalist)
'robot's hearts set on first' is the wordplay.
'robot's hearts' becomes 'obo' (I can't explain this - if you can you should believe this answer much more).
'set on' says to put letters next to each other.
'first' becomes 'ist' (ist resembles ost).
'obo'+'ist'='OBOIST'
Can you help me to learn more?
(Other definitions for oboist that I've seen before include "Instrumentalist" , "Do I boost the musician?" , "orchestra member" , "Wind player" , "Musician who plays woodwind of treble pitch" .)