Herald's up on one leg, out of control (7)
I believe the answer is:
rampant
'out of control' is the definition.
(I've seen this before)
'herald's up on one leg' is the wordplay.
I cannot really see how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rampant that I've seen before include "Showing unchecked growth" , "On the rampage" , "Out of control" , "Unbridled" , "Flourishing excessively as disease may be" .)