Deputy's also scrummy occasionally (5)
I believe the answer is:
locum
'deputy's' is the definition.
(I've seen this before)
'also scrummy occasionally' is the wordplay.
'occasionally' means one should take alternating letters.
'also'+'scrummy'='alsoscrummy'
The alternate letters of 'alsoscrummy' are 'LOCUM'.
(Other definitions for locum that I've seen before include "Doctor's stand-in" , "Stand-in professional" , "Temporary substitute" , "Stand-in physician" , "One stands in" .)