Home ground, partially — in Derby , say (7)
I believe the answer is:
habitat
'home ground' is the definition.
(I've seen this before)
'partially in derby say' is the wordplay.
'partially' becomes 'a bit' (I've seen this in another clue).
'in' indicates putting letters inside.
'derby say' becomes 'hat' (derby is a type of hat).
'abit' going inside 'hat' is 'HABITAT'.
(Other definitions for habitat that I've seen before include "Territory" , "Living environment" , "Environment in which groups normally live" , "Natural home and surroundings" , "Animal's natural home" .)