Long jumper doesn't want strong field (3)
I believe the answer is:
lea
'field' is the definition.
(I've seen this before)
'long jumper doesn't' is the wordplay.
'long' becomes 'l'.
'jumper doesn't' becomes 'ea' (I am not sure about this - if you are sure you should believe this answer much more).
'l'+'ea'='LEA'
'want strong' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for lea that I've seen before include "Open arable land (poet.)" , "Tract of pastureland" , "'The lowing herd wind slowly o'er the . . .'" , "Land growing grass" , "''The lowing herd winds slowly o'er the ... '' (Gray)" .)