Asleep so mates guarding Derby town (5)
I believe the answer is:
'derby town' is the definition.
(I've seen this before)
'asleep so mates guarding' is the wordplay.
'guarding' says the answer is hidden in the clue.
'EPSOM' can be found hidden inside 'asleep so mates'.
(Other definitions for epsom that I've seen before include "Poems about the race-course" , "& 5 Purgative medicine" , "Surrey race course" , "Town with racecourse" , "Derby area" .)