Laundry used to be hard (4)
I believe the answer is:
wash
'laundry' is the definition.
(synonyms)
'used to be hard' is the wordplay.
'used to be' becomes 'was' (eg 'he was...' means 'he used to be...').
'hard' becomes 'h' (abbreviation used in pencil classifications).
'was'+'h'='WASH'
(Other definitions for wash that I've seen before include "Lave" , "Stand up" , "Rough water left by a moving boat" , "shower" , "The ----, inlet of the North Sea" .)