Fancy Hector’s share? It’s the scorer’s job (13)
I believe the answer is:
'it's the scorer's job' is the definition.
(writing musical scores)
'fancy hector's share?' is the wordplay.
'fancy' indicates anagramming the letters.
'share?' becomes 'ration' (a ration is a share of a limited resource).
'hectors' anagrammed gives 'orchest'.
(Other definitions for orchestration that I've seen before include "Musical arrangement" , "Composer's skill" , "Note arrangement" , "Stage-management" , "Arrangement for instruments - short reaction (anag)" .)