Seasonal farm-worker, one hastening to acquire garment (9)
I believe the answer is:
harvester
'seasonal farm-worker one' is the definition.
I can't tell whether this definition defines the answer.
'hastening to acquire garment' is the wordplay.
'hastening' becomes 'harer' (I can't justify this - if you can you should give a lot more credence to this answer).
'to acquire' indicates putting letters inside.
'garment' becomes 'vest' (vest is a type of garment or item of clothing).
'harer' enclosing 'vest' is 'HARVESTER'.
Can you help me to learn more?
(Other definitions for harvester that I've seen before include "Reaping-machine" , "Long-legged spider" , "Farm worker" , "Crop gatherer" , "One gathering in crops" .)