Cut down on one part (4)
I believe the answer is:
reap
'cut down on one part' is the definition.
'reap' can be an answer for 'cut' (I've seen this in another clue). I am not sure about the 'down on one part' bit.
'cut down on one part' is the wordplay.
'cut down' means to remove the last letter.
'on' becomes 're' (regarding).
'one' becomes 'an' (an apple is one apple).
'part' becomes 'p' (this might be a standard abbreviation I've not previously seen).
're'+'an'='rean'
'rean' with its last letter removed is 'rea'.
'rea'+'p'='REAP'
Can you help me to learn more?
(Other definitions for reap that I've seen before include "Save harvest, glean" , "Cut or gather (a harvest)" , "Harvest (a crop)" , "Derive as advantage" , "Gather 23dn" .)