Cleansing tractor, horse and lorry (9)
I believe the answer is:
cathartic
'cleansing' is the definition.
(thesaurus)
'tractor horse and lorry' is the wordplay.
'tractor' becomes 'cat' (cat is a kind of tractor).
'horse' becomes 'h' (synonyms).
'and' means one lot of letters go next to another.
'lorry' becomes 'artic' (articulated lorry is a semi-trailer truck).
'cat'+'h'+'artic'='CATHARTIC'
(Other definitions for cathartic that I've seen before include "releasing suppressed emotions" , "Releasing pent-up emotions" , "providing a release" , "Cleansing - purifying" , "Type of drug" .)