Unbalanced when mounted? Best not say it then (5,2)
I believe the answer is:
giddy up
'best not say it then' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'unbalanced when mounted?' is the wordplay.
I cannot really understand how this works, but
'unbalanced' could be 'giddy' (giddy can mean excited or dizzy) and 'giddy' is present in the answer.
'mounted?' could be 'up' (up on a horse) and 'up' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for giddy up that I've seen before include "suffering from vertigo?" , "attempt to get horse moving" , "On mount get moving" , "Gee!" , "that's said to mount" .)