They throw balls in vessels (8)
I believe the answer is:
pitchers
'vessels' is the definition.
(I know this)
'they throw balls' is the wordplay.
I cannot quite understand how this works, but
'throw' could be 'pitch' (pitch is a kind of throw) and 'pitch' is located in the answer.
The remaining letters 'ers' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for pitchers that I've seen before include "playing baseball?" , "Players" , "American sportsmen" , "Water-carriers" , "Ewers" .)