Period engaged in some busy admin returns (4)
I believe the answer is:
days
'period' is the definition.
(I know that days is a type of period)
'engaged in some busy admin returns' is the wordplay.
'engaged in' indicates a hidden word.
'returns' is a reversal indicator.
'syad' can be found hidden inside 'some busy admin'.
'syad' back-to-front is 'DAYS'.
(Other definitions for days that I've seen before include "See 7 and 18" , "Sun, Mon, Tues, ..." , "` `They are not long, the .... of wine and roses' ' (Dowson)" , "See 25" , "See 1 down" .)