Yes indeed; under 50 per cent (3,4)
I believe the answer is:
not half
'yes indeed under 50 per cent' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'yes indeed under 50 per cent' is the wordplay.
'yes indeed' becomes 'not' (I can't justify this - if you can you should believe this answer much more).
'under' says to put letters next to each other.
'50 per cent' becomes 'half' (I've seen this before).
'not'+'half'='NOT HALF'
Can you help me to learn more?
(Other definitions for not half that I've seen before include "A full pint?" , "thoroughly" , "I couldn't agree more!" , "Certainly!" , "Very much (colloq.)" .)