Laundry used to be hot (4)
I believe the answer is:
wash
'laundry' is the definition.
(synonyms)
'used to be hot' is the wordplay.
'used to be' becomes 'was' (eg 'he was...' means 'he used to be...').
'hot' becomes 'h' (abbreviation, eg on taps).
'was'+'h'='WASH'
(Other definitions for wash that I've seen before include "shower" , "Bear investigation" , "Rinse" , "wake" , "Clean; thin coat of paint" .)