Hard for us for some time (5)
I believe the answer is:
hours
'some time' is the definition.
(several hours)
'hard for us' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'for us' becomes 'ours' (I've seen this before).
'h'+'ours'='HOURS'
'for' acts as a link.
(Other definitions for hours that I've seen before include "time appointed for work" , "A long time" , "Matins and Vespers, say?" , "Periods of time" .)