Time's hard for us (4)
I believe the answer is:
hour
'time's' is the definition.
(unit of time)
'hard for us' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'for us' becomes 'our' ('our' means belonging to us).
'h'+'our'='HOUR'
(Other definitions for hour that I've seen before include "(Period of) time" , "Length of time" , "When it cometh, so doth the man!" , "Three thousand and six hundred seconds" , "Period of day" .)