Old agricultural worker picked up white horses (4)
I believe the answer is:
surf
'white horses' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for surf that I've seen before include "Waves breaking on the shore" , "It's created by breaking waves" , "Randomly look at" , "Visit various websites" , "Balance on waves" .)