Trim until they're lean (4)
I believe the answer is:
tilt
'lean' is the definition.
(synonyms)
'trim until they're' is the wordplay.
'trim' means to remove the last letter (I've seen 'trimmed' mean this (the final letter is trimmed off)).
'until' becomes 'till' (I've seen this in other clues).
'they're' becomes 't' (this might be a standard abbreviation I've not previously seen).
'till' with its final letter removed is 'til'.
'til'+'t'='TILT'
Can you help me to learn more?
(Other definitions for tilt that I've seen before include "Engage in a medieval joust - lean" , "Tip to one side" , "Inclination, joust" , "Lean to one side" , "Cause to slope or incline" .)