A field of runners is so energetic (4,2,5)
I believe the answer is:
full of beans
'energetic' is the definition.
(I've seen this before)
'a field of runners is' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is found within the answer.
'of' could be 'o' and 'o' is located in the answer.
'is' could be 'be' and 'be' is found in the answer.
This explanation may well be incorrect...
'so' acts as a link.
(Other definitions for full of beans that I've seen before include "Energised" , "after feast in seedy restaurant?" , "Might maracas be" , "like a cowboy after a meal on the range?" , "Eager and alert; full of high spirits" .)