Time hard to manage when upset (5)
I believe the answer is:
epoch
'time' is the definition.
(epoch is a kind of time)
'hard to manage when upset' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to manage' becomes 'cope' (manage can mean to cope or do well).
'when upset' is a reversal indicator (in a down clue - set, written, up).
'h'+'cope'='hcope'
'hcope' back-to-front is 'EPOCH'.
(Other definitions for epoch that I've seen before include "Particular historic time period" , "Historical period" , "Period, era" , "Major division or period of history" , "Period of notable events" .)