They know their actions mean the high jump (12)

'they' is the definition.
Both the definition and answer are plural nouns.
Perhaps you can see an association between them that I can't see?

'know their actions mean the high jump' is the wordplay.
I cannot quite understand how this works, but
'mean' could be 'par' (I've seen this before) and 'par' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?